On a pretty much random basis, Outlook decides that a .pst file is being used by another program, and it won't open it.
Typically I can switch from one .pst to the next - until the 12th file - then it says "no". However, it will open some additional files, but not others. 12 seems to be a "magic" number.
Also, when I only have my inbox open, such as when I am moving e-mails into archive files, it will say that certain .pst files are unavailable. Yet, if I shut down Outlook and then restart, and select the errant .pst file, it opens just fine.
One error is
can't move the items. the set of folders could not be opened
I did detect & repair which did not resolve problem.