Not primary error writing data frame to MongoDB

Question

I cannot write any data to my MongoDB using R mongolite. I consistently get: Error: not primary error.

collection <- mongolite::mongo(url=my_url, db=my_db, collection=my_collection)
collection$insert(iris)

Error: not primary

This Error: not primary message occurs whatever I try to insert.

The database works fine, as I can write to it using Python without any problems.

I would appreciate any help!

• Tried other data frames, JSON objects, lists, ...
• Tried inserting a hand-written JSON string, like this: collection$insert('{"a": 3}')
• Tried if I could find in the same database using mongolite (yes)
• Tried if I could write using Python (yes)
• Checked Mongolite documentation and github