Well I tried flashing the full decrypted RUU and got the same failure as before. After debating with myself for a while on what I should try next, I decided to flash a Sense HBOOT like Scotty suggested and then try to reflash the RUU. I was a bit concerned because if the full RUU that contains the proper HBOOT wouldn't flash, maybe a zip with just the HBOOT.img won't flash either. But I figured I need to just give it a try.
I downloaded the ATT_OTA.4.18.502.7.firmware.zip & extracted the HBOOT.img & the android-info.txt. I edited the mainver: to 3.17.502.3 since I leaving all the other firmware the same. Zipped those two files & flashed w/o a problem. I then attempted to flash the full decrypted RUU. Again I got the same error.
O:\Android\HTC_One\RUU>fastboot oem rebootRUU
(bootloader) Start Verify: 0
OKAY [ 0.047s]
finished. total time: 0.047s
O:\Android\HTC_One\RUU>fastboot flash zip RUU_M7_UL_K44_SENSE55_MR_Cingular_US_4
target reported max download size of 1514139648 bytes
Invalid sparse file format at header magi
sending sparse 'zip' (0 KB)...
OKAY [ 0.156s]
(bootloader) zip header checking...
(bootloader) shift signature_size for header checking...
FAILED (remote: 32 header error)
finished. total time: 0.344s
So now what? I figure I'll download the official signed RUU and try to run that. But I'm having a hell of a time downloading files from the HTC1guru site. All the files I've tried to download end prematurely. I don't know what the issue is with that, if it's on my end or the server. Anyone got another source where I can download RUU_M7_UL_K44_SENSE55_MR_Cingular_US_4.18.502.7_R1 0_Radio_4T.24.3218.09_10.26.1718.01L_release_35656 5_signed_2.exe?