Jens,
thanks for your report. The interesting issue is that this particular computer is my childhood one. At the time (read late 80s early 90s), I had purchased an external floppy disk drive that worked great with my system with a 512k memory expansion and a 50MB external hard drive in the sideslot.
I remembered now back then that at some point during the mid 90s, the external floppy drive stopped working, basically as soon as you'd plug it in, the motor was constantly on. At the time, being a kid, I just didn't know how to work with electronics so I left it at that.
Fast forward 20 years where that computer just sat at home while I was abroad, I decided to get back into the A500 and purchased the ACA500plus to make it more accessible today. It's now that I tried again to plug the external floppy again, and of course the same issue happened, so I decided to throw the drive away and replace it with a Gotek. Lo and behold, the Gotek gave me the same issue, but at first I didn't put 2 and 2 together (when I started this thread).
Now that I know that the issue was "fixed" after swapping CIAs, I guess that floppy drive was good all along, and I dumped it for nothing.
Just thought I'd give you the full history behind this.