@momokass - I am experiencing the same behaviour when I try to export a frame as an image. I will report it to Miro support and update this post with what they tell me.
In the future, you can find the support contact form by searching for “contact form” in this article (there are also steps on how to load the support form from your Miro dashboard):
Thanks for getting in touch with us and reporting the issue! We're truly sorry you've run into it.
I'm afraid that currently, there's a bug on our end that causes this behavior. The good news is that the team is already aware of the issue and is currently working on a fix for it. We're bringing our sincere apologies for all the inconvenience it causes!
We'll definitely keep you updated on the progress. Please, bear with us!