LimeSurvey version:   LimeSurvey Community Edition   -   Version 5.3.12+220502
Own server or LimeSurvey hosting: Own server 
Survey theme/template: Fruity - extended theme. Small design changes undertaken.
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Hello All,
The button to view the public statistics in the survey throws an "error 500" when clicked.
I have reset all the changes that I have made to the extended theme for the purpose of debugging, but the error still pops up.
Thankful for any help to debug this error.

Before starting any debugging I always recommend an update o the latest version. Your's is now over 7 months old and there might have been quite some bug fixes in between. Who knows and what is causing this has been fixed in the past 7 months already.

If it still persists, then I think it makes sense to check if this is a general bug or something specific to your installation.

What you can also do is check with another survey if this survey also throws the same error.

And has it worked before and doesn't work now or are you not sure if it worked before? Because the "Column not found: 1054 Unkown column 'language' in 'where clause'" doesn't sound it could be a caused by a theme but is rather a database issue.

Thank you for the reply @holch.
I have contacted the IT team regarding the update.
But regarding the other questions, the button still works in other surveys and has worked well in the past.
I honestly don't know what might have triggered this issue.

If it is working with other surveys, than it is most probably an issue with this specific survey. Difficult to say what could trigger the issue. But you can try to copy the survey and see if the problem persists in the copy of the survey. Or export and import it again and see what happens.

Also check the data integrity of the survey. There seems to be obviously an issue with the column "language". Maybe also check if you can find something about this column "language" in de database table for this survey.

Thank you for the reply @holch.
I checked the data integrity and all options meet the consistency standards. I furthermore checked the database, but there isn't any other language column other than the base language column.
But that just reminded me that I added more language to the survey while developing it in the beginning but then removed all of them before activating the survey. But since there is no other language column in the database I am not really sure where to exactly find an debug the issue.
 

Some times there are "residuals" left of some previous action, which could well be the case here (languages not deleted completely, or something).

Sometimes, these things "magically" disappear when you copy the survey. Or when you export and re-import it. So give that a try.