I had the same problem reported here, but none of this solutions works from me.
- php.ini "file_uploads" is ON
- tested changing permissions for all files and folders to 777
- moved wiki location from subdomain to folder
- entered "basedir" manually
- changed gdlib from "autodetect" to "Version 1.x"
After a bit more research it turned out the problem is caused by overly-restrictive security policies set in the webserver. Find out more about it here:
https://www.dokuwiki.org/faq:mod_security
- tried disabling mod_security with .htaccess-a "SecFilterEngine Off", but then got "500 Internal Server Error" on all pages
Finally I've resolved the problem by contacting hosting provider and pointing them to the previous document. At the bottom there are some suggestions about rules that should be disabled.