Acronis Management Server is installed on-premises on a Windows machine, and built-in SQLite database is used.

You want to manually clean up the Activities log.

1. Download and unpack SQLite Precompiled Binaries for Windows (select the 'bundle of command-line tools for managing SQLite database files').
2. Open Command prompt as Administrator: Start -> in search, type cmd -> right-click cmd.exe and select Run as administrator.
3. Navigate to the folder where the extracted sqlite3.exe is located.
4. Issue:
   sqlite3.exe %ProgramData%\Acronis\TaskManager\task_manager_db.sqlite3 "delete from activities where state = 'completed' "

The Activities logs should be deleted.

(!) If the machine is rebooted or Acronis services are restarted, Activities log will be restored, because Agent synchronizes with Management Server. This is expected behavior.

Additional info:

If you need to clean up only a single activity which was stuck for example, please refer to https://kb.acronis.com/content/60653