I’m using Collabora Online App Version 4.2.2 with Nextcloud 21.0.4.
When I create a new office document and choose the “empty” (german: “Leer”) template, it results in a 0 Byte size File, which opens as plain text file with the writer. Regardless whether I requested a text document, a spreadsheet or the power point equivalent “presentation”.
This behaviour is new, since I updated from the last 20.x version this evening. The collabora online app was working as expected until then.
Uploading additional document templates which are empty but valid documents is a workaround, but the chance to use the wrong template remains.