OOS and Nextcloud --- Powerpoint error, however Word and Excel work

After the Installation of NC 21.0.1 and Windows Server 2016 plus Office Online Server 2019
Word and Excel work fine.
But Powerpoint does not work and this Error occurs:


I applied an Lets Encrypt Certificate.

What can i to do? thanks for help!!


Thank you Steel_PC for your help up to now.

It seems that the NC installation, wopi 3.5.7 are fine and the OOS Installation make the problem.

Now i try if the cert of lets encrypt may lead to this problem.

How should it be configured?
Should it be the same for the OOS installation and NC installation?
Is an Lets Encrypt in general possible, it must be published by Lets encrypt?