I keep getting this error, but the stack sizes had been working.

Troubleshooting steps I've taken:
Confirm the StackSizeController.json files in both Config and Data folders passed validation on https://jsonlint.com/
Deleted .cs file from Plugin, and .json files from Config and Data, then rebooted server.  Reinstalled .cs, rebooted, then copied json  information into Data folder.  Multiple times, both with and without server being stopped.
Removed plugins installed by server owner after I'd installed the stacksizecontroller mod, and confirmed it worked - no joy.

I'm at my wit's end - are any mods known to interfere with SSC, or does this sound like funky serrver host hardware (GameServerKings)?

Thanks in advance for any input anyone may be able to provide!

Post the config and data files and maybe someone can spot the error , the message you get regarding corrupt file might also give a clue (?)

This appears to be fixed - I found an error with a plugin that seemingly has zero interaction, fixed that error, then stack sizes started working.   I'll reconfirm it's still working tomorrow, and delete the thread if it's still okay at that time.