I'm not talking about fairly new DCS versions, I'm talking old, old missions like some originally written for BS1 or FC2. Also, the error occurred in the log file when saving the mission not at run time. This sounds like it's a different problem.
A couple of things you might check: 1) If you're using DCS 1.5 or 2.0 make sure you're not using a 'mission start' trigger. It may be fixed now, but it was broken for these versions. 2) If you are copy/pasting from notepad++, try different encoding settings, for example try UTF-8 or UTF-8-BOM.