Using the REST API filter, including two meta_queries breaks the response for one custom post type

Oddly, the cause of the problem seems to be the quotation marks around the comparison parameter.

For example filter[meta_query][0][compare]=%27=%27 now needs to be filter[meta_query][0][compare]==.

When only matching one meta value, this is not necessary, when matching two, this is, now, necessary. :/

Update: I realised that my hosting provider changed the version of Linux on my server around the same time. I can imagine this might be connected.