I solved it.
Short explanation: dumb
Long explanation: duuuuuuuuuuuuumb ;)
I messed up the gateways. In the LAN interface the IPv4 upstream gateway was set to the wrong gateway.
Setting it to "auto-detect" solved my problem.
I do not remember if I set it or why, it may be in the beginning where I tested to fit OPNsense in our infrastructure.
Short explanation: dumb
Long explanation: duuuuuuuuuuuuumb ;)
I messed up the gateways. In the LAN interface the IPv4 upstream gateway was set to the wrong gateway.
Setting it to "auto-detect" solved my problem.
I do not remember if I set it or why, it may be in the beginning where I tested to fit OPNsense in our infrastructure.