Embeds fail sometimes, {{unknown}} in postmeta, how to debug?

[rmens]

Sometimes embeds from Instagram and Facebook fail without a clear reason.

When checking the postmeta, there is no html in the postmeta, just {{unknown}}. When deleting all the _oembed_* values from the postmeta and trying again, the embed sometimes works and valid html is showing in the postmeta.

I suspect the API is returning an unknown response, but this is not visible anywhere. Can you give some guidance on how to debug this? For example enable a 'debug' mode that shows the api response. Thanks!