Bug? Or user issue? "is not an XCA database" when trying to open XDB file

[Mike55555]

I have been using XCA successfully for years - editing my database about 6 times a year. Today I tried opening it, and I am getting an error " is not an XCA database". I am using version 2.9.0, but also tried v2.6.0 - same error.
So I thought perhaps the file is corrupt --- so I went to my backups and restored 4 different versions from the last few years. All of them get the same error.

If I create a fresh empty XDB file, I am able to open it successfully without any problems.

What am I doing wrong? Or is there a bug?

If it makes any difference, I always install XCA using Chocolatey. [choco install xca]