When (in power automate cloud) I put 3 pptx files there (2 of them from encodian populate actions and one from sharepoint get files directly) it does not work (empty 500 error). However, when I put only the output of the encodian populate actions it works fine.
To be clear, this works:
But this does not:
From what you have described your issue is being caused by feeding two different formats of file content into the action i.e if you look at your inputs you will likely see something like this
On their own both of these are valid or (as you have found) if you only pass Encodian files or only sharepoint files the action works but you cannot mix the two formats.
In order to resolve the error wrap the sharepoint file content in a base64 expression
In this case using this expression returns only the base64 string.
Once this is done the action will run as expected.