The short version:

Do not unplug or plug in a mc4 connector under load. Meaning do not unplug when solar panels are actively powering something on the circuit. The odds are high you will damage the connector and it will likely not be able to handle the current it could previously before melting. It is very very important to not disconnect mc4 connectors or connect them to something that will immediatly be powered.

If you want to be extra careful, it is best to not mix and match mc4 connectors from different manufacturers unless you verified that the two manufactures work together. It sounds strange but you can find

The long version: