This issue can occur when you are running the openconnect in dtls mode which is by default and it can halt the connection completely. It is related to MTU size of the connection and size of the packets. in order to fix it, you should decrease the MTU of the connection until this error goes away. Just add the following option to openconnect config file or command line (of course the config file doesnt need two dashes) :
--base-mtu=1450
if you still get the error message, keep decreasing the 1450 value by 50 points until this error goes away.