I can login to http://www.dokuwiki.org/
from my iPad running 6.0. If it is a caching issue, how could we test for that, possibly:
- nuke safari's cache and see if login becomes possible (I haven't replicated the issue so can't try this yet).
- check server log files to see if the post request was received on the server. I have checked the POST traffic between dokuwiki.org & both my iPad and an iPad emulator (also 6.0), it all looked normal I'll compare against iOS 5 on the emulator later (need an update to get it working again).