What is error 335545108?

I have set up a new PC and installed MSH again. When I start MSH I get this error:

Unable to open the master database in C:\Users\lesro\OneDrive\Documents\MailStore Home. A database error has occurred. Code: 335545108. Details: Invalid crypt key

I can’t find this error code in the knowledge base or this forum. What does it mean, and how do I fix it?

This means one of the .key files are missing or corrupt.

This can happen when MailStore Home is stored in a directory where files are monitored or modified by synchronization software such as OneDrive or Dropbox. The fix is to restore the matching .fdb and .key files from backup, and going forward, do not store MailStore in such a directory (you can copy the files into OneDrive, but the live database should not be run from OneDrive).