I assume both of the Electrum clients are on the same version?

It's hard to tell which part of Electrum is throwing that error and what is causing it. Could you go to Tools>Preferences and check "Write Logs to File". Afterwards, restart it and do the same QR code signing again. When it throws an exception, go to your data directory (usually %appdata%/Electrum for Windows) and go into the Log folder. Open the log file with the latest edit and check the contents for the exception. There should be certain files and lines referenced. Omit all the sensitive information (shouldn't be too much) and share only the specific part.

I can't replicate your error no matter how many times I've tried.