If you see this error message when trying to log in to the LanguageTool Thunderbird add-on:
“Oops! Seems you couldn't submit form for a long time. Please try again.”
it usually means that cookies are disabled in your Thunderbird settings. Cookies need to be enabled for the login process to work properly.
How to fix this:
Make sure cookies are enabled in Thunderbird
If you’re unsure how to do this, check Thunderbird’s support resources or preferences related to privacy and cookies
Once cookies are activated, try logging in again.