yes, I rebooted the PC a few times yesterday and it was off overnight. Same situation today.
The files save perfectly when FS2004 is not running, so currently I am back to pre 2013 procedures - run FS2004, see what must be done, close FS2004, carry out whatever work has to be done on the gauges in question and save the .xml files, load FS2004, test to see if the work carried out was exactly right (seldom!), close FS2004, carry out whatever work has to be done to rectify the coding problem and save the .xml files, load FS2004, test to see if the work solved the issue and so on. To think I used to do this all the time before this thread started! I am going insane here!
I recently had a crash on my external backup disk and then on my 'extras' HDD where all my flight sim utilities, downloads and documentation were held. Both have now been replaced and I am rebuilding up towards the situation which was there before the crashes. Despite the hassle it caused me, I do not believe the HDD crash can be responsible for the situation here now. (It would however be nice if you could point to a problem there and get me working again )
Nothing interfered with the system disk which is an SSD nor the FS2004 disk, another SSD - I have carried out no work on either (except for changing panel.cfg and gauge .xml files).
A recent Windows 10 update has resulted in the same scenario for C-gauges. I used to be able to change Situations, recompile the gauge and switch back again. No longer - the compiler complains that it can't access the relevant gau/dll. I now have to close down FS9 / FSX, recompile and restart. Oddly, P3D still behaves in the old way.
I installed KB4457128 on 14th September. Due to my disk crash I have not been in a situation to carry out edits recently. Perhaps the problem has existed quietly since then and I am only copping on to it now.
I hope someone comes up with a solution, and soon!