Good morning all together,
i updated the Limesurvey manually from our 2.50+ to the recommened Version 3.22.19+200605. In general it worked, but we have issues with the preview off our studies. If we press it, there is always a blank page. It always show this path: domainname.com/limesurvey/index.php/741245?newtest=Y&lang=de (which is blank), but i already tried to reupload the index.php. What can i do next to fix the problem?
Thank you very much for your suggestions.

What PHP version?

In application/config/config.php, set debug to 1 or 2, retest and see8 if errors are displayed.

Our Webhoster says we have php 7.2, so this should be quite updated.

I set debug to 1 and there actually came a full side of Errors. This one was on the very top: Class 'Twig\TokenParser\SetTokenParser' not found. Do you need more of these informations?

I found the solution. I checked this path and actually this specific data was just not there. I compared it with my Limesurvey-Download-Package and in there it was. So i just uploaded this file via filezilla and now it works.
Good Tip in general: Set debug to 1, so you can see the error and take a look for this file. Then compare it to the original downloaded file and reupload this one. Sometimes not everything gets uploaded correctly.

This is a common problem with FTP. If there are a lot of files to be transfered (as it is with Limesurvey) it can easily happen that not all files are uploaded. Not a LS problem itself, this can happen with any FTP upload, in my experience. Especially as the LS installation files seem to increase and the uploads are getting bigger, I have changed my approach. I upload the whole zip file to the server, unzip it there and then move the files to the right folder on the server. Less chances that a file doesn't get transferred.