Hello Markus,
Happy New 2019 Year!
Is GVRP enabled on SG550?
To check issue the command:
switchxxxxxx# show gvrp configuration
What is the switchport mode on SG550 links - access or trunk ?
For example lets say you have uplink gi1/0/1 - issue the command:
switchxxxxxx# show run int gi1/0/1
Cheers,
Mike