@Juraj_Budai - I found the issue for your dataset. The annotation characters did not get parsed correctly.
The box labels are expected to be strings led with a letter (and we’ve recently added support for labels to be led by numbers, if you choose).
In your case, I assume you uploaded a dataset that contained non-English characters for the labels, or you added non-English characters for the labels within Roboflow.
Modify Classes will be required to remap the characters in your generated version so it comes out successfully: