Felix,
Gotcha. That's kind of strange.
There are 2 things you can try to get access back to the WAP.
1. Try Internet Explorer. This is most likely due to SSL error on older firmware.
2. Factory reset your WAP
From here, you can try these things:
1. Check your licenses. Do you have room to add another WAP to the ZD?
2. Try another R500 if available, to confirm the issue
3. Upgrade your ZD to higher version. It can go as high as 9.10. Be aware that if you have WAPs that are way to old like 2942s or 7341/7143s, they will stop working based on the version. Read RELEASE NOTES before upgrading.
Cheers,
Alex