In your old message, you also mention getting the following error:
Error 500: View not found [name,type,prefix] JFBCONNECT,HTML,JFBCONNECTVIEW
Are you seeing that message anymore? If not, then it's not an SEF problem or any other conflict. That message was why I referenced the SEF extension.
If the only issue you're having now is the un-translated strings for:
COM_JFBCONNECT_THANKS_FOR_SIGNING_IN
COM_JFBCONNECT_EXISTING_USER_INSTRUCTIONS
Please check your language files for JFBConnect. Those strings should be in the language file you're using, like /language/el-GR.com_jfbconnect.ini . They will look like below:
; COM_JFBCONNECT_THANKS_FOR_SIGNING_IN="Thanks for signing in using %s. Do you already have an account on this site, or would you like to create one?"
COM_JFBCONNECT_EXISTING_USER_REGISTRATION="Σύνδεση"
; COM_JFBCONNECT_EXISTING_USER_INSTRUCTIONS="If you already have an account, just log in here, and we'll link your %s account to it."
There is a semi-colon ( ; ) in front of each of those lines, which means that the string isn't translated, and that the English file should be used instead. I'm not sure why the English files aren't being loaded properly, but the best fix is to:
* Remove that ; from each line
* Correct the translation on the right-hand side for your language.
I already have an account on your site, so I couldn't try to go through the registration process. Hopefully that helps get you going though. If you want me to test, please delete my account on your site.
Thanks,
Alex