The problem has gone away. It turns out that the problem was caused by
hitting a limit of either the allowed number of lines of code attached
to the Userform or the size of the Userform code module itself was
greater than a certain limit. I don’t know for certain.

I suspected that something was corrupted, but it appears that that
wasn’t the case. It looks like this message occurs when you hit some
sort of undocumented limit. So by going back to a previous version,
not the actual previous version, but two levels back which worked and
by splitting the code into separate modules, I can add more code
without a problem.

There doesn’t appear to be much original information on the Internet
about Excel VBA limits. However I did find:

http://www.mvps.org/dmritchie/excel/slowresp.html - mentions a soft
limit of 64K per module and a limit of 4000 lines of code per module.

I did find some information in the following (excellent) books:

(1) Professional Excel Development (Wiley) - mentions a soft limit of
64K per module – page 45.

(2) Professional Excel Development: The Definitive Guide to Developing
Applications Using Microsoft Excel, VBA, and .NET (2nd Edition)
(Wiley) – also mentions a soft limit of 64K per module – page 43.