I'm also encountering these errors. Seems related to the migration to the new “Marketplace”?
I’ve upvoted your post and hope someone from Mendix sees this. I won't create a ticket for Mendix Support as of now because Rik already said in his post that he has created a ticket.
Was this problem already sorted out for you Fabian? I wanted to update one of my widgets and got one of those errors as well. I also think updating an existing widget has become more cumbersome than it used to be..
Regarding the “ Unable to save changes at this time. Please try again later. “ – One thing I’ve noticed to fix that error is making sure that the source file you upload doesn’t contain any spaces in its name. Not sure if this error message occurs only for that type of error.