The charger doesn't charge or shuts off — what's the cause?

What came up in the chat:

• the magnetic connector inside the charger came loose — open it and check
• overheating in hot weather — protection switches it off, half an hour later it works again
• if the bike charges with someone else's charger, the charger is faulty: repair or replace (the compact one costs the same as the big one)

The stock charger is waterproof with the cover closed.