I've never faced this issue, but I would suggest you mark the following folders as exceptions from AVG (AVG can lock files, which is what causes MCEBuddy to fail conversions as it checked for locked files during conversions).
- All folders where the recorded files are kept and MCEBuddy is monitoring them / where you will add manual files to the queue
- All destination / output folders
- Temporary working directory (AVG should NEVER lock a working file)
- Installation folder