@Jutta, this is a hard question to answer. Many unknowns.
What you may do is to understand the error.
WordPress may have the error log.
Set define( 'WP_DEBUG_LOG', true ); in your config file, and then you need to check the results as per this article.
Note that this allows you to write to /wp-content/debug.log
A web server may have the error log.
Work with your Hosting support if WordPress error log cannot help.
This is not strictly WordPress item, so I will be a fish in there.
You can work on both these to understand your problem better.
Please update your question with more details once you find that.
You may even consider accepting my answer if you like it, or if you think it is helpful, because this is a general approach what to do. We may create additional questions for specific errors we may find on your web server.