I am trying to import different (.iss, .isa, .txt) format into limesurvey (different versions - paid version too) but it gives me an internal error and it does not work. 
Tested on Version 3.22.1+200129, Version 3.26.2
 

Attach a sample survey (.lss file) here.

here is an example file but i tried other example files too already and none of them works.

can you provide me an example file from your end to make sure if there is nothing wrong with my account specifically? Thank you.

I have no problem importing the survey. What error do you see?

Here is a test from my end -  

www.limesurvey.org/manual/Debug_mode and try to import.

Thanks for your reply. The errors i am getting while exporting the survey file is attached.

Also the survey is displayed in the list of surveys and I try to click on it or delete it: it says: 500: Internal server errorSelected Surveys language not found An internal error occurred while the web server was processing your request.Please contact administrator to report this issue.

So did you check if the baselanguage (or any language of the survey) is avaiable in your Limesurvey installation?

yes it is but it does not work

Ok : try to do a www.limesurvey.org/manual/Check_data_integrity on your system.