You messed up the data file while editing.

it gets the error every time the server is rebooted, this is tested by removing the files from config. data and plugins. Then placing a new stack size file into plugins. setting new stack size in game via chat (works until reboot) , If you reboot the server, it hapens again.

How is the server being restarted? If a control panel is killing the process, that can corrupt files.

I do a restart with the nitrado cp. I can try to do a full stop and start and see if anything changes

Yeah, that is most likely a force kill of the server process then, so it doesn't get a chance to gracefully save... and could also be saving while it does that, which is what would corrupt the files.