Thanks @laurens and @arjanvanb for sharing the “context” workaround, and at the same time bringing to our attention an issue when posting the position as a tuple {"lat": 52.395316, "lng": 4.874904}.
Both requests are equivalent. The problem was we recently deployed a new version of our ingestion layer, which was causing an issue when using the position tuple. As soon as we saw your note, our team deployed a fix and now things are back to normal.
As a recap, these two formats are valid and equivalent in Ubidots API:
{"position": {"lat": 6.2, "lng": -75.4}}
or
{"position": {"value" : 1, "context":{"lat":-6.2, "lng":75.4}}}