Loss of Connectivity when disconnecting some switch

[gary_the_network_guy]

There is something unexplainable happening in my network. After disconnecting a switch from my router, I cannot reach all devices connected to the device. It is a very simple network, with one core-switch, two access-switches, one transit-switch and one MPLS router. After disconnecting a unnecessary link to another network, this whole thing does not work.

[MHM Cisco World]

can you share the topology ?

[gary_the_network_guy]

Sure, here is the topology, as far as I know it. The thing is, it is an inherited network and there is no documentation.

[gary_the_network_guy]

I think there is a server connected to the core, which is also connected to the old infrastructure.

[MHM Cisco World]

that can be, remove uplink and keep downlink can lead to this case. 
only check the show mac-address in OLD see of Server MAC still show in table. 

[gary_the_network_guy]

I now configured the ports connecting to my infrastructure as access ports. still after removing the link to the mpls router to the old core, there is no connection. Maybe the transit switch is configured wrong?

[gary_the_network_guy]

The old server is now only connected via access port, so no trunking.