Synchronet has supported up to 16-character internal codes for many years now, but GoldEd+ never caught-up (still limited to the 8-char internal codes used many years ago). I don't use this product, but thought I would should at least file a bug report since it appears no one else has filed one regarding this long-standing problem (though they've been telling me about it for years).
To reproduce the problem, the sysop needs to have an "internal code prefix" set for one or more of their message groups in SCFG->Message Areas. Once this is done, GoldEd+'s msgs.cnf (Synchronet configuration) file parser fails to get the correct internal code for the sub-boards within that group.
The fix is easy and I'm going to send a patch in.
-Rob