I got a Codesys 3.5.16.20 runtime on a PLC communicating with a SCADA platform.

We keep loosing connection to the PLC and the PLC log inside Codesys IDE has a lot of the following lines

warning: CmpOPCUAStack, 05-02-2021 10:01:58 - OpcUa_TcpListener_ReadEventHandler: Process Request returned an error (0x800B0000)!
error: CmpOPCUAStack, 05-02-2021 10:01:58 - OpcUa_SecureListener_ProcessRequest: Closing channel due error 0x800B0000!
error: CmpOPCUAStack, 05-02-2021 10:01:58 - OpcUa_Endpoint_BeginProcessRequest: Not able to create/send response. (0x800B0000)

or

warning: CmpOPCUAStack, 05-02-2021 09:30:09 - OpcUa_TcpStream_Flush: Error writing to socket: 0x80B50000!
warning: CmpOPCUAStack, 05-02-2021 09:30:09 - OpcUa_TcpStream_Flush: Error writing to socket: 0x80B50000!

or

warning: CmpOPCUAStack, 04-02-2021 10:40:31 - OpcUa_SecureStream_Flush: Flush 0 failed with 0x80AD0000! (Last)
warning: CmpOPCUAStack, 04-02-2021 10:40:31 - OpcUa_SecureStream_Flush: Could not flush transport stream! Status 0x80AD0000!
warning: CmpOPCUAStack, 04-02-2021 10:40:31 - OpcUa_TcpStream_Flush: Error writing to socket: 0x80010000!

But where can I find out what the hex error codes mean?

They are defined by the standard, so I usually just Google for OPC UA 0x80010000

If it is unencrypted then wireshark is useful for showing which request caused it to fall over.