How about listed what the error you get?
Some more information definately is needed here as this is a bit vague about the specific errors not listed here.
1. This could simply be a simple PHP memory issue if your uploading a larger file. How big is the file etc. What category on your install? How about posting a test upload account for me to try uploading a file otherwise I can register a name but figured I would ask since most people already have a test account made.
2. No issues here tagging or untagging index thumbs so exact message please.