I didn't quite understand on the wiki page.
DHCP server is connected to ether1, access points are connected to ether 2-5.

Now my question: do I only have to activate "trusted" for Ether 1 or also for Ether 2 to 5, where access points (wlan) are connected?

On "bridge" I have DHCP Snooping activated.

From DHCP Snooping article: In RouterOS you can specify which bridge ports are trusted (where known DHCP server resides and DHCP messages should be forwarded). In other words, use trusted=yes on the port that is facing towards DHCP Server.

If you are using multiple bridges with DHCP Snooping and Option 82 enabled, you also need to mark trusted ports which are going to receive DHCP messages with already added Option 82, otherwise these messages will be dropped.