Trying to upload calc template, "there were some errors"

trying to share a calc template, but keep getting a “there were some errors” message.
Everytime I fill out the data on one of the tabs and hit “save” I get the error, and the screenshot and source file gets removed.

Tried it with Chrome and Edge.

Any ideas whats wrong?


Found out my mistake, wrong file extension (spreadsheet ods → template odt).