I’ve found what is causing this error, and share it here if anybody is in my situation.
In fact the problem does not come from my database neither from my network … but from my client server.
Sometimes my client server get its CPU overloaded, all cores going to 100%. In those cases i guess it can’t handle in time its mongoDB requests which leads to this MongoPoolClearedError.
Anyone having this error should check on client side if the server is not overloaded.