OK, I have found the solution myself, or near as I can to one.
One of you on another thread here tipped me off that it should be an evaluation version of 10.1, not just 10.1
Therefore, did a search and this page shows you how to install the evaluation version (someone in the UK has kindly set up a page for retailers to refer to, a big thank you to them!)
8 weeks of frustration ended, and hopefully others Googling the web will find this solution fast.
Test and mine is now playing most Youtube videos (probably won't play all of them if they are too new, I'm not sure)
Spoke too soon, not resolved. It played two videos then back to square one, even with those same videos. Getting blue cubes again. Will try uninstalling it, rebooting and reinstalling. If that doesn't work, will look for caches/data to clear in either the browsers or other software and see if clearing those works, like it does when I clear the cache for Market to re-enable the ability to download apps. Will update here if any progress.
update 1 (all testing using stock browser called My Browser as don't have any other Flash-capable browsers available to download, Youtube HQ off)
Got it working on some videos again by doing a force stop on My Browser via Manage Applications. Certainly won't play all videos, perhaps if they were made for newer Flash versions. Not sure how a user can tell which Youtube videos will play other than by trying them.
(Note - when it does play a video can take at least 7 seconds to start playing it)
Some videos on Youtube - clicking for them to start results in them loading and then without any user intervention they do not play and the page reverts to YouTube Home.
Using the force stop method, managed to play another shorter video from YouTube, loading time was 4 seconds. However, it would not then play again when I pressed for a replay. Refreshing the page then led to a blank window of blue in the browser. Seems very ropey at the moment.
Getting "invalid response received" or "Taking too long to load?" messages presumably from Youtube on occasion
Best results for getting a video to load from Youtube
i. Close My Browser and do a Force Stop via Applications->Manage Applications
ii. Start My Browser again
iii. Go to Youtube, search for the videe you want via text search and select it from the thumbnail, pressing the arrow on the screen if your Plugins are set to "On Demand"
iv. If it won't play, try another user's upload of the same video
Must stress that at the moment I can't say it's solidly reliable, and it is tedious to keep going and doing a Force Stop all the time and reloading My Browser. It might be that clearing the cache/data on My Browser would also work or help more, but that also clears bookmarks etc from My Browser. I was unable to find a program that worked and memorised my bookmarks so I try not to use Clear Cache etc (also accessed from Manage Applications)
Am also wondering if I get more reliable Player use/activation with the battery nearer full charge or plugged in to the mains power.
Note the videos play completely jerk-free, smooth, no problems at all, when they play.
EG Just played Timbalake/Carey/Rosette's "Amnesia" from two different uploaders - absolutely fine. [uploader: Dragseba99] 4/5 videos I got playing didn't even stop to continue downloading - which is better than my laptop!! Weird, it's the same connection except my laptop uses ethernet and the tablet wi-fi.
But then the very same video then won't play!
So either the Force Stop does some good, or it's perhaps a case of the device not having enough memory to reliably work continually between loads of data. I recall that usually erratic results are sometimes low memory issues.
Have just done another Force Stop and going back to Amnesia (appropriately named!) to see if the same video that played before then didn't play will now play again. No, Dragseba99's didn't this time, but another upload of Amnesia by someone with a name in Cyrrillic letters did ('TV' in a circle logo top right of screen).
However, Lana Del Rey's Videogames (her own upload and the vevo ones) won't load and sometimes when I click for it I am returned to the home page automatically - that's even with the Force Stop technique.
Then Amnesia wouldn't play again, etc!
So it is a bit of a head scratch.
Might be something to do with the cache or memory.
Lot of patience needed - or just buying a better tablet. In which case, a thorough test in the shop on their wifi viewing lots of videos, closing the browser, then viewing more - all necessary before sinking your money into another tablet. And of course, something else might then be wrong with the new one. On the plus side, this does most things quite well and my home wi-fi works faultlessly with it since it came back from the repairer. Videos play very smoothly. I guess for a cheap device, I will stick with it for now as its eccentric behaviour is usually just limited to the Flash Player side of things and it performs well on the rest.
If I find out how to get Flash 10.1 to always work without hassle, I will post.
Cleared the browser cache and date, has NOT helped.
A clear of the cache etc means the UA string and other options reset.
(The UA string should be set to "Android", Plugins on.)
Tried Skyfire 2.3 and 4 - it hangs, and won't start
NextVid won't play any YT videos although as a program seems good
Tonight I have been unable to get the device to play ANY YT videos!
Sort of kinda back to square 1 or 1A!
Did a Force Stop again and got some videos to play - the same videos
that wouldn't play earlier. However, some videos such as Videogames on
YT still don't play.
Am thinking it might be a case of the hardware being short of the necessary spec - memory or chip. It could be that when there is enough memory available, Flash will load. Otherwise not.
Closing as many other programs as possible might be they key to getting Flash to work.
I'm not sure the 256MB RAM DDR2 in this unit is enough. Not sure if it can be done, but a swap file to boost the memory may help. If I get anywhere, I'll update again. Right now, the best I have is erratic results - sometimes it works and sometimes not.
AFAIK Android is Linux-based, so these might be the minimum requirements for Flash 10.1 Evaluation (or 10.1) to work reliably on a device, info from Adobe's website
- 2.33GHz or faster x86-compatible processor, or Intel Atom 1.6GHz or faster processor for netbooks
- Red Hat® Enterprise Linux® (RHEL) 5.6 or later (32 bit and 64 bit), openSUSE® 11.3 or later (32 bit and 64 bit), or Ubuntu 10.04 or later (32 bit and 64 bit)
- Mozilla Firefox 4.0 or Google Chrome
- 512MB of RAM; 128MB of graphics memory