Hello André
I'm trying to debug a fiber connection between two datacenters in Zurich.
We got a 1Gbps "dedicated Layer2 Connection incl. Managed Switch" from Init7.
As a first step you probably want to fact check if you have a "darkfibre", which might be measured with a power meter and/or OTDR, or if you have a layer-2 service as you worte above.
A layer-2 service means there is some active gear (usually from your service provider) in between you and the physical fibre between the datacenters which they manage and therefore you can not optically measure the line as a whole.
If I put everything together, I still see a "notconnect" on the switchport.
As a simple test: see if you see have no light (down/down) at both ends of your connection. My guess: it's only at one end and you most probably deal with a defective optic (GBIC/SFP) at some point...
Bottom line: most probably you will have to deal with your service provider.
Good luck.