Thank you for the quick answer!
I started a "naked" vbulletin-installation within mamp from my local harddisc with nothing more than this addon and all links to external sigpics worked flawlessly!
So I was aware of having to deactivate and activate some of my many addons on my live-forum to dig further into this issue. For some reason I just activated "Ajax Threads" again without deactivating any other mod. Surprisingly now all external signature-links are working too on my live-forum!

So disabling and reenabling "Ajax Threads" seemed to do something magic, what is far beyond my noobish explanation-facility. Anyway, I?m glad, that it works now.