Please help us help you and fill where relevant:
Your LimeSurvey version: Community Edition Version 6.2.6+230904
Own server or LimeSurvey hosting: Own Server
Survey theme/template: Fruity 23
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I have recently installed LimeSurvey and set up my first survey (I was using an older version so I was familiar with all the concepts involved).
I Activated the survey and went through it myself, but when I submitted the survey, it gave me the following error:

"500: Internal Server Error Call to a member function decryptBeforeOutput() on null"

I have NO idea what this means. Can someone help?

Do you have an email notification activated in that survey? So that you would receive a email when someone submitted and survey?

Yes, it is going to two email addresses. I separated them by commas... should I use semi-colons?

Please try the survey without any email notification.
If the issue will be gone, I recommend to reenter all the mailserver credentials in the mail settings and save again.

Then reactivate the notification and try again.

Personally I don't recommend using notifications at all.
If there is any issue with the mailserver (e.g. timeout or slowness), the survey of everyone try to submit a filled out survey will be at slowed down or fail.

You should update LimeSurvey 6.x as well, if there is a newer release.