what does the following line in my debug log means?

PHP Deprecated:  substr(): Passing null to parameter #2 ($offset) of type int is deprecated in /home/deb111392/domains/expeditionmeister.com/public_html/oc-includes/vendor/gettext/gettext/src/Utils/StringReader.php on line 31

It pops-up 100 times a second or more....

attached the file containing line 31

I think you are using PHP 8.1 and osclass version is not yet ready for that. If you use PHP 8.0 the error should go away

Correct, we are using 8.1

However, if i change it to 8.0 my website doesn't load and gives me an empty webpage...

How to fix this?

That is very strange, is a fresh installation using default theme?

I am using a child theme of the Delta Theme, but it is strange that 8.1 works and 8.0 not...

@Hugo
Something is wrong most probably. I suppose if you enable error logs, both will be on same.

I am sorry, i don't understand what you mean..

Attached a screenshot of the errorlog

@Hugo
Can you read it?
Enable error log.
https://docs.osclasspoint.com/debug-php-errors

sorry, see the new screenshot

We know that this error is caused by PHP 8.1
I suggest you to delete the log file from server, change PHP to 8.0 and then access your website that show blank page. After that, check the new log file, so we can see what is the error using PHP 8.0

This error is most probably related to PHP 8.1, before null in this function was just fine.

I debugged my website while I used PHP 8.0 as recommended; please check the following screenshot of the outcome

Thank you!

Unfortunately; i am not that experienced yet... could you explain what i need to adjust?

Thanks!!!! <3

Check if you have mysqli extension enabled in your php.
Discuss with hosting provider.
That function should not be missing.