I've run into the same problem, and also discovered the 911-character limit.
Since it's in an existing application, and we've been running it with the
same data that cause the problem (a string of about 1871 characters), it's
likely that some fairly recent Office update has caused it.

Note that it doesn't happen in VBA; I can start Excel and put a 2K-character
string into a cell with no complaints. It's when I'm working at a lower
lever using a SAFEARRAY of VARIANTs that I get the problem.

Don