Yeah, apparently v4 firmware for the S7-1500's is off to a rocky start. I haven't repro'd, but allegedly with v4.0.4, "The PLC may terminate the connection when two simultaneous connections read data larger than approximately 1300 bytes per request at short intervals."
If the connection is dropped abruptly while we're in the middle of reading, you'll likely see errors similar to this thread: Siemens Symbolic disconnect after adding certain tags