Problem with my server or with the software?

I've got this addon for my XF site that's not working right and causing all kinds of problems. The addon's author is saying I have a problem with my server, but the developers on the XF forum are saying it's a problem with the addon's code.

So obviously I'm in a strange situation. I don't want to accuse the developer of being wrong, but on the other hand I have about five sites on this server right now and none of them are having any problems, and I don't have any problems outside this one addon.

Here's a screenshot of the error log:

 

Could be a file permission issue.

 

Would that be with my server?

I've also been having problems with FTP access. Could that be related?

 

Okay, so it doesn't appear that the permissions for the file in question are the source of the problem. Could it just be that the php in that file has an error in it?

 

E_WARNING is a problem your script developer should be able to help you with
The "Undefined variable" kind of issues are issues of the script i think, but i do not know for sure. Are your web hosting server using PHP version that your script developer recommending? Can your hosting provider change the PHP version for you?
What about trying to backup your files and then re-upload the addon/script (i mean the one that is causing troubles, not whole content management system probably) if it fix the issues? I would overwrite already existing files during upload while making sure i indeed have original backup of the files i am overwriting.

 

This seems to be a problem with improper permissions or php handlers. Try to check with a deferent handler.