DNN core: Community v5.5.1

After years of trouble free use of the DropDownActions menu for modules I now have the situation where most of the drop downs items in the DropDownActions menu are blank (they just appear with an underscore instead of "_Settings", for example).

One other problem (which may be related) is that I am no longer seeing confirmation messages DNN places on a page (e.g. when you successfully change a password for a user you would typically get a 'Password successfully changed' message) .

This strange behaviour has started despite not installing any new modules in several weeks, nor have I upgraded any existing ones or the DNN core itself. the only thing that has been done is I cleared up a very large backlog of schedule history that reduced the databse size from 211MB to about 145MB. Could it  be related?

I have tried clearing the cache and restarting the app but that has not fixed it. I am at a bit of loss for troubleshooting this. Any ideas out there on where else I should be looking?

Thanks.

Eoghano