This issue along with the "Catastrophic failure..." errors being posted in the RDB Sync log can occur when using an older version of Access RDB Sync with a newer version of Microsoft 365 Outlook.
The below steps are for downloading and installing the latest release (6.5.0.500) of Access RDB Sync, for use with Access RDB Pronet v6.7 and Microsoft 365 Outlook.
⚠️ Important: This procedure involves installing software, so you will need suitable privileges on your PC to be able to complete it.
Download and Installation Procedure:
Close RDB Sync if it is running on this machine.
Check and note the "bit-wise" version of Outlook installed on this machine (from Outlook, go to File, Office Account, About Outlook, and note from the long version number whether this application is 32 bit or 64 bit).
Download the zipped installation package (RDB Sync - 6.5.0.500 - Version Increment.zip).
Find the zip file after download and right click on it, then subfolder RDB Sync - 6.5.0.500 - Version Increment.
Go into the subfolder folder that corresponds with the "bit-wise" version of Outlook noted in step 2.
Install RDBOutlookSyncSetup.msi.