Thanks, we have now a very rough regression range 2016-04-01 (68a8f4989e6) (works) – 2016-09-08 (897a70c5954) (fails) which amounts to more than 460 commits to choose from and have ruled out locale as a factor. Now it is up to the Author to find some time (or not to find any time) to look into it. The issue is IMVHO minor but somewhat exotic.
If you later manage to further reduce the range and to check the exact Qt5 version in Avidemux2.6.app/Contents/Frameworks/QtCore.framework/Versions/5/Resources/Info.plist, it would not harm either.