Hello everybody,

I have a hex router, behind it I have a local network that is 10.10.3.0/24.
Eth 1 is WAN
Eth 2-5 is in a bridge.
I have a DHCP range from 220-254.
If I connect my PC to it I get an IP-address (10.10.3.249) and can reach the internet.
I can also give my PC a static adress of 10.10.3.8 or 10.10.3.10 and still reach the internet and I can also ping my PC from the router.
However! If I give my PC a static adress of 10.10.3.9 nothing works, I can't ping my PC and I can't reach the network at all.

Being a bit used to things breaking in Windows I simply connected Mikrotik switch and gave it the 10.10.3.9 address and that didn't work either but it also works on 10.10.3.8 or 10 or whatever adress in the span I give it. Whaaat???
It's like I'm not allowed to use 10.10.3.9

I only have one network on LAN and the other network is the WAN. They are not overlapping in any way, not even close.

Cleared ARP, disabled all other interfaces in the bridge except eth4 where my PC is plugged in but it didn't help.

I can reset the router and start over but that way I won't learn what I did wrong .

Either you have something in router config targeting explicitly the offending IP address ... or you have another device on the network with said address configured and then it causes MAC address conflict which severely disturbs data flow. So far it's impossible to tell if either of my theories have any merit as you didn't provide configuration export (and we can only guess about another device residing on said IP address, you'll have to try to find out if this might be the case).

With all other interfaces disabled, it cannot be an IP conflict (duplicate IP by static or dynamic setting), unless it is something within the PC you are using (but that makes little sense).
Ideally you should try with another PC/laptop/any other wired device, this would exclude also the "client" side and the only possibility would remain a (mis-) configuration of the Mikrotik router.