Right Alt Key (Alt Gr) only working every second time

Hello,

I bought the dual monitor KVM switch (HKS0402A1U-EUBK) around 4 years ago. I used it for a while but then switched back to a single computer setup so I didn't need it for a while. Now I switched back to a two-device setup (one Mac, one Windows device) and therefore set up the KVM again. 

Now after using it for a few days, I recognized that my right alt key (alt gr) is not working properly. I have the following problem:

When typing special letters such as “@” (right alt + l), the letter is only printed every second time I press this combination. Every other time, it just won't print anything. Also, in the case it does work, it spams the letter (e.g. “@”) until I release the right alt key, even if I release the other letter (e.g. “l”) before.

I checked on karabiner elements event viewer, and indeed every second time I only press the right alt key the input is not recognized. What is interesting is that if I hold the right alt key in the scenario where it isn't recognized, as soon as I press an additional key to the right alt key the right alt key press is recognized, but it doesn't result in printed letters, only if I keep holding the right alt key, release the other pressed key and press a different key.

This is really frustrating, because there is no way for me to correct this behavior via any kind of karabiner-element or keyboard software, and I do need the key quite often.

Do you have any suggestions, firmware updates, or configuration changes that could help resolve this behavior?

Kind regards