Hello there Did you try to reinstall that version of Thunderbird? Most of the issue s that combined this issue with Thunderbird are the most common. Are insufficient disk space. Other programs could be interfering the main program Thunderbird. Other software programs could be interrupting the possible action to use the compacting function continuous in that Thunderbird version. If are there more issue s that block or infect the software and seems to react corrupted , then it is possible to change the index file.msf this could be fixed by deleting this file. It rebuilds it itself again by restarting Thunderbird. Other subjects might be there , that there is not a possibility to compact is what kind of antivirus is used there? Is that antivirus to heavy to use with that program are there scans with the software, can you see if compacting is possible again when antivirus is turned off? Very large folder has to be shortened into normal folder capacity. These subject s can stop that Thunderbird function to compact your folder or folders. Reporting this as a bug? Can you send me a screenshot of that message ? https://support.mozilla.org/en-US/kb/how-to-make-screenshots Beta version s could not be total complete to use already. And could be solved by developers at any time , you can use the full version. And try to download from Thunderbird trusted point on the internet. Questions are open. Greetings googlethunderbird.