Hello again,
same issue, but different root cause.
I noticed, that we still sometimes get this TCL-error in the logs.
As we are using a "default" iRule, which is in place on several VS, I checked all of them to verify if there is maybe another iRule in place, which performs a redirect.
I didn't found such an iRule, but other iRules doing an "IP whitelisting" and if not matching, TCP-connection will be dropped. I guess the behavior is similar to a redirect. The TCP-connection isn't available anymore for HTTP header insertation.
I will try to move the HTTP header insertation iRule on the very top of all VS.
Ciao Stefan :)