Since it is normally the little things that I find are capable of jumping about a meter to bite me in the hinder parts, those are the ones I tend to start with. To wit,

• are uploads permitted on both?
• do both have the same upload size limit (if such a limit is set)?
• do the permissions compare on the temporary and upload directories in the webserver configuration? in the filesystem?
• how do the file sizes compare between the successful and unsuccessful tests?

Don't know if any of those suggestions will help, but good luck in finding your solution. HTH.