iRule to rewrite cookie domain to toplevel domain

When you do not use the if statements, you insert/replace domain cookie because you do not question about $cookieDomain value. That's why it's working.

The if statement are not working just because there is nothing into $cookieDomain variable. So the they always return "fasle" and you end up into the final else (the one logging Not a single cookie domain matched :/)

It would help if you could post HTTP headers into the response (name and value, eventually values anonymized) since it seems you want to rewrite domain cookie in responses. You can use developer tool in Chrome or Firefox to do that (F12 and network console), or a tcpdump or any other solution you like.

I have the feeling you're not using the correct cookie statement (ie. HTTP::cookie domain in your iRule) but I can be wrong. Having clear listing of HTTP headers in responses would be definitively helpful.