Is it safe to reheat food in plastic containers

As this basically got answered through questions in the comments, I'll collect up all of the various problems that might help prevent the sogginess:

1. After frying, allow the croquettes to cool on a wire rack, so they don't get moist as they give off steam.
2. Reheat first at a lower temperature to heat through, then turn it up the temperature to regain the crunch & get any extra browning.
3. Make sure when reheating that they aren't packed too tightly, and if possible, heat on a wire rack rather than a pan.
4. If reheating in a small but tightly-sealing oven, try cracking the door slightly so as to prevent humidity buildup.

(anyone who has any other suggestions, feel free to add 'em)

There are plastic oven bags like this

Source http://lamiacucina.wordpress.com/2014/02/20/saumon-en-papillote-au-citron-vert-lachs-in-folie/

that can be used at temperatures at least up to 200°C. I am sure that there should be equivalent baking pans with similar properties for baking goods.

In conclusion, referring to your link to another question: It depends on whether the plastic is supposed to be baked or not. The questioner of the other thread said he had forgotton to remove a piece of plastic that he had been supposed to. In your case: It is very likely safe.