When I try to log in I get the following error

I have actually the same problem ...
Same here; Windows 10 x64.
Solved by deleting %APPDATA%\yakyak, launching the app and signing in afresh.
Just tried partlyhuman's fix, and it worked like a charm. On Mac, the command is rm -rf ~/Library/Application\ Support/yakyak/
Instead of deleting your app data, you can also go back to the first screen where it shows you the account you're trying to log in to, then click "Add account" below that, and log in normally using the same account.
Solution also works for Linux, remove ~/.config/yakyak
Instead of deleting your app data, you can also go back to the first screen where it shows you the account you're trying to log in to, then click "Add account" below that, and log in normally using the same account.
This did not work for me back when I first started banging my head against this, and today it did. Thanks for motivating me to try it again.
Most helpful comment
Same here; Windows 10 x64.
Solved by deleting
%APPDATA%\yakyak, launching the app and signing in afresh.