Thx, for all replies
it doesn't have to be in the sdext2 directory (this is the mounted ext2 formatted partition created by link2sd by the way) I also tested it in other directory under different name but all attempts failed.
ES file explorer refuses to create a folder "anywhere" in the root, I can create files everywhere I want just fine but not folders. I can also rename en delete folders anywhere I want.
Personally I'm thinking the create folder option in ES is the culprit, but why does it let me create folder on the regular SD partition (music for example)
Normally logical thinking solves many things, but it does not make any sense anymore.