I'm getting the following message when trying to upgrade from 2.50+ to 2.51.3+

The following errors were found :
Required PHP extension not found : json
Required PHP extension not found : mbstring

And when i'm trying to activate adminpanel or a survey i'm getting the following message:

Internal Server Error
This script needs the PHP Multibyte String Functions library installed: See <a href=' www.limesurvey.org/manual/wiki/Installation_FAQ '>FAQ and PHP documentation


Can anyone please help me with this problem.

Thanks in advance!

Well, your PHP installation on the server is missing the JSON and the MBString extension.

It looks like the Limesurey comfort updater isn't warning about missing things and changed requirements in terms of PHP versions and PHP extensions. Looks like only the fresh installer does that.

You'll will need to check the version of the installation PHP and might have to update. To fix the issue you need to add the missing PHP extensions. The way to get that done differs. It depends on the operating system and the specific setup of webserver and php.
The administator of the webserver should be able to fix that.