Thread:Project:Support desk/Could not create directory after upgrade attemp from 1.15 to 1.20/reply

It's likely to try to create the directory $wgUploadDirectory/temp/1/1e. Be sure that $wgUploadDirectory and all its descendants are writable by the webserver.

If in doubt, give all permissions, check if that solves the problem, and then tune up permissions until it's secure and working.