Error: "Cannot export file using given character encoding"

[abasil96]

When performing a merge, I recently started getting this error when I try to save the result file in the DiffMerge GUI:

Cannot export file using given character encoding. Unknown encoding (-1)

This prevents me from completing a merge in ClearCase. The source files being merged are UTF-8 encoded as far as I can tell via Notepad++.
Merges used to work fine for years, and now they suddenly don't.
The workaround involves a rather painful process. It would be nice to know what's causing this problem.

Some useful version info
OS: Win 10 Pro 22H2, build 19045.4651
DiffMerge: 4.2.0 (697:52f98c4fcb) [x64] stable

[Tonya]

Hello,

We have a few questions:

1) The problem appears to be file specific? Or does the issue happen for all files? If it's file specific, does the issue happen for all files of that same type?

2) You said the problem recently occurred, what changed?

3) What steps do you take to resolve the problem?

Thanks,

Tonya