Thanks, but been there done that ... happily, I've returned to post the solution to my own issue in case it may help others in future: the issue was a small program I've been using without issue for some time, but which apparently has a conflict with the later versions of .Net framework — I recently upgraded .Net from 4.5.2 to 4.6.2 and I believe that is when the issue began.
The program I refer to is "ArchiveInvalidation.txt File Generator" created for Fallout New Vegas (according to its readme.txt), which relied on .Net 4.0/4.5 — I will inform it's creator of the issue shortly and I dare say it will likely be fixed promptly with an update once he is aware of the issue.
My apologies for any inconvenience caused by posting here and...