We have an Informix 11.70 with multiple instances, but
I receive the error "27001: Read error occurred during connection attempt."
when I try to connect one specific instance, if I change to other instance I get connected.
recheck step by step the releationship between $INFORMIXSERVER as env variable, check its presence in the RIGHT $ONCONFIG file, check its presence in sqlhosts file, that the protocol is correct and that the service name (if tcp) is not closed.
If all is correct, may be the connectivity files in the /INFORMIXTMP directory are outdated. When offline, check this file is not present.