1. Find out what MT5 thinks your data folder should be.
PrintFormat("Data directory is [%s]", TerminalInfoString(TERMINAL_DATA_PATH));
Does it exist? Does it not exist? It's an ontological question.
2. If it doesn't exist, create it. Or restore it from a backup. You may have to reinstall.
3. If it does exist, try rebooting. @kypa's suggestion of 7-zip is interesting.
I'll keep that in mind. Thanks for the tip.