Re: Errors when uploading large files
Please see here
https://dev.tiki.org/item7038-CSRF-Error-when-trying-to-log-in-from-the-top-bar#threadId=7314
This is classed as a bug and I understand it should be fixed soon.
I don't know if that fixing that bug will resolve your specific problem, but it may be worth tracking that ticket and possibly add a few comments cross-referencing this issue you encountered.