I have already been googling to find what this means and had no luck. I can not find any settings for TCP flags in Net_Config_TCP.h nor do I know what file the HTTP error could mean. Any ideas?
this error happens, when receiving a TCP FIN, or RST, or ACK packet at an unexpected time. Use wireshark to see why you received such an unexpected TCP packet.
Note that TCP is a "handshake" protocol. There is a well-defined state machine: http://www.tcpipguide.com/free/t_TCPOperationalOverviewandtheTCPFiniteStateMachineF-2.htm#Figure_210
The problem is suddenly gone. Unfortunately I can't say why exactly.