Today my MiG-21 module stopped working too.
Wanted to launch a quick mission and ended up in an error message, telling me the serial is invalid.
It surely isn't. The module worked fine since September 17th, 2014, where I've bought it through DCS online store.
No hardware changed, and all drivers and DCS up to date.
I also tried to reinstall themodule, but got the same result.
Sent an email to support@... but wanted to file this issue here too, as it's the only module not working for no reason.