Hi Günter Grauberger,
Greetings. Thank you for posting the thread to our community.
I understand your concern, it seems the Find and Replace menu has been changed into side pane. For the situation you’ve encountered, I do understand the inconvenience caused and feel very sorry about it.
After my test, I’m afraid and sorry to say that this situation also happens on my side. Here is my test result as reference for you:

In that case, since you mention it used to be a pop-up dialog box, I suggest you can revert your PowerPoint version to an earlier version:How to revert to an earlier version of Office - Microsoft Support and check whether the scenario will change back or not.
If the situation persists after reverting multiple versions of PowerPoint, I’m afraid this setting has been globally deployed to the all the version of Office and this situation needs to be reported to related Microsoft development Team. I suggest you can try the following methods to report this issue:
1. Use the in-app feedback tool to report this issue and send your **feedback for PowerPoint:**Go to Help > Feedback > Click Report a problem to report the issue:

2. Create a free support ticket and contact Microsoft Support to report the issue: You can create a free support ticket to report the issue as Microsoft Support team has higher permission and latest resources for Microsoft products. You can contact the support to get latest update for this issue.
If you are an admin, I recommend you get support by following steps from this link: Get support - Microsoft 365 admin | Microsoft Learn.
If you are not an admin, you can contact your admin to contact Microsoft support. You can look for the way to find your admin through this link: How do I find my Microsoft 365 admin? - Microsoft Support.
Please accept my apologies for redirecting you to the related development team support, as the moderators in this community have limited resources and permission to verify the backend information and to get the quick and better assistance, we requested for this.
Appreciate for your patience and understanding, and Hopefully Microsoft will make improvement soon.
Hope you having a nice day.
Sincerely,
Harry | Microsoft Community Moderator