There have been a number of reports of users getting the message ' Upload server error; please try again later ' when trying to upload attachments.
This occurs when you exceed the 100MB space allocation of a consumer Google Site. So the first thing to check - if you are using a consumer Site (i.e. not a Google Apps Site) is are you close to the space allocation limit of your site. Go to More>Manage Site>General and look under Site Storage. If you are close to the limit, try clearing out some files and try again. Remember that deleting an attachment 'soft' deletes it and it will continue to occupy space for 30 days or until you 'hard' delete it by going to deleted items and selecting 'permanently delete. (Which could explain why some users have found this an intermittent problem - deleted items being removed after 30 days frees up space for the user to now be able to upload more files.
Two other suggestions which some users have reported as sometimes working are :
In the meantime there are two things users can try that may help.