I have following setup:

A Mikrotik with a bridge-local with ether1 and wan1, and a dhcp server(different machine) connected to ether 1.
When i connect clients to lan1 they get an address from the dhcp-server but when i try to connect a client to wan1 it cant connect because it
doesnt get an ip. It works if i assign a static ip for a wlan client.
Is there anything i need to configure on the wlan interface?

On which interface is running the DHCP client?
This must running on the bridge interface.

the DCHP Server is not running on the mikrotik routerboard.
It is a windows server connected the lan interface of the bridge.

First- Have you bridged eth1 and wlan1
created a bridge add ports wlan1 and ether1
Two- setup DHCP-client on the bridge that you made
Thats all

Thanks for the answer but you are misunderstanding me.
The DHCP Server for our LAN is NOT configured on our mikrotik.
My Problem is that Clients connected to the LAN-Port of the Bridge(the DHCP-Server is connected there too)
get a DHCP-Offer, but on the WAN-Port of the same bridge, they dont.

Thanks for the answer but you are misunderstanding me.
The DHCP Server for our LAN is NOT configured on our mikrotik.
My Problem is that Clients connected to the LAN-Port of the Bridge(the DHCP-Server is connected there too)
get a DHCP-Offer, but on the WAN-Port of the same bridge, they dont.

WAN-port (ether port where Internet cable is plug-et) or WLAN-port (wireless card)
u can tell as what kind of board are u using

anyway by default config there are some firewall rules u can disable them and try

WLAN Port of coures .

Can you give a bit more explanation as to what you are trying to do here.
There are two options either you trying to setup a dhcp relay or you running both the dhcp server and the wlan clients on the same lan in which case you need to bridge the ports. If you can give us more info we can better assist you.

Setup a dhcp relay for the network you are wanting to use on the wlan and select interface wlan. Server should be the ip of the dhcp server.

Okay, First Problem is solved.
Someone left an old router with the same ssid plugged in, wich caused the strangeeffects.
Now dhcp works if connected to the mikrotik over wlan.

The Problem is i can only connect if i disable any security.
If i activate Wpa/Wpa2 i always get
wlan1: disconnected, unicast key exchange timeout

anyone had the same problem?