@Emilie Vermeulen - I have not experienced this personally.
Have you tried deleting deleting objects from the frame either one-by-one or in chunks, to determine if it is a specific object or the frame itself that is causing the issue? Note: If you do attempt this, I would of course recommend making a copy of the entire template as a backup (basically adding the template to an existing board or by creating a new board from the template).
That being said, even if you do identify the problem object, this should still be reported to Miro support – the support form can be found by following these steps:
https://help.miro.com/hc/en-us/articles/360020185799-How-to-contact-Miro-Support#h_01F010RXA0E1S9C0WFQYX75R6C
Just ran into this and figured out what may have been the problem.
I am not company admin, but I was trying to save as a template to make it accessible to the whole company. So I followed the instructions in the save template dialog to “invite a company admin to the board and ask them to save it as a template for the company.”
When she tried this, it didn’t work, resulting in the vague error above shared by the original poster.
What worked was not just adding her to the board, but also adding her to the team the board was created in. Once she was added to the team, she could save it as a template for the company successfully. So if you’re running into this issue, I would not assume that things will “just work” for a company admin. Double check the teams they’re associated with as well.