As a general rule, when I receive a request to referee a paper from a journal, I do my best to accept it unless I have a strong reason not to. One possible reason is that the journal in question is predatory or otherwise morally objectionable (emphatically, this does not include rigorous journals which are just not particularly strong). Usually, it's easy enough to tell a predatory journal from a legitimate one, and a moderate amount of googling tends to resolve most remaining cases. However, I've just run into an edge case I find difficult to resolve, and I was hoping that people wiser than me have already done their due diligence and can share the result.
I'm specifically interested in the MDPI journal Mathematics. It definitely seems to be a real journal publishing papers that are not obviously nonsense and include DOIs. For what it's worth, it even seems to be open access. They have a Wikipedia page, which does not outright call the journal predatory, although it has a section on "Evaluation and controversies" which seems to be longer than all the other material combined. The paper which I received seems to correspond quite well with my fields of research, which is usually quite a good sign, but the deadline sounds unreasonably short.
Any advice would be appreciated!