The new feature in v.3.6.2 which tells you which file is preventing a folder from locking is a brilliant success as far as I am concerned. It consistently highlights the fact that the obstacle which has been preventing my Mail folder from locking is com.apple.internetaccounts. Force quitting this allows me to lock Mail.
Is it likely that my com.apple.internetaccounts file is corrupted - and if so is the solution to delete it and then recreate all the accounts and settings?
I cannot comment on that, it is Apple's scope, not ours (Espionage).
I would first check which process is keeping that file open and then decide how to proceed.
If the process is Apple's and I assume there is a good chance for that, then Apple Support is the only one who can say something here, unless you find something similar on forums or google in general.
Rgds
Zsolt