Please help us help you and fill where relevant:
Your LimeSurvey version: Version 5.5.0
Own server or LimeSurvey hosting: Server of my university
Survey theme/template:
==================
Hi there,

I have set up a survey with four question groups and in two of them, there are videos the participants are supposed to watch in order to answer the questions. The order of those two question groups is randomized.
I started the testing phase of the survey and half of my testers had no problem at all while the other half was unable to play the video. The people who couldn't play the video tried it on different devices and the link they got was exactly the same as the one the others got. There is no common factor of those two groups - both include people using Andriod and Apple devices.
Has anyone got an idea what the problem might be and how to fix it? I'd be really gratefull for a quick answer as I need to start the survey soon in order to finish in time.
Have a lovely monday,
LM

edit: my file was corrupted and after re-rendering it, it works now.

With your description it is basically impossible to say anything.

Is it always the same video? Or are both videos the problem?

What means they are unable to play the video? What happens? Any error? How is the video implemented?

Do you have a link? Do you have a LSS file for us to look at?

Otherwise: no idea how you want us to help you.

It's like calling the mechanic and saying: My car sometimes works, and sometimes doesn't. What should I do?

Not that I think that it has to do with your issue (as this actually doesn't look like a LS issue, but rather an issue of the video or its implementation), but your Limesurvey Installation 5.5.0 build 221219 was released on 19.12.2022, so it is almost 6 months old. I recommend an update.

But seeing that it is a university installation, there is probably little you can do. Actually, for a University installation this is surprisingly up-to-date (not that this is really a good thing, just not as bad as others). 

Thank you nevertheless! It was actually my mistake because the file was somehow corrupted and re-rendering it has resolved the issue
Have a lovely day
LM

also - should I delete this entry now that it has been resolved and if yes, how do I do that?
Thanks a lot!

No, you can leave it. You probably can't even delete it yourself. But you can leave it how it is.