Since the Hardened Mode is off by default, why not try knocking it back to Moderate rather than aggressive.
Set something up to be aggressive and it is going to be, well very aggressive, though the add to exclusions should resolve that unless it relates to other sandboxie fires, etc.
Have you tried manually adding the sandboxie executable file to the Hardened Mode and file shield exclusions ?
avastUI - Antivirus - Exclusions, the File Paths & Hardened Mode.