Generic Errors in data_to_read but everything works fine if we ignore that

[Bascy]

We are communicating from an ESP32 with a HomeWizard Battery, using Ethernet and SSLClient.
The data_to_read function regularly returns a Generic Error (-1) and when this happens, SSLClient::available() will stop() the connection. After some fiddling about we discovered that when we do not stop but continue (comment out stop() on line 458) everything works fine.

Why do we get the Generic Error when everything seems to be working fine?