@D-C I double checked everything, and I have somehow managed to get the multi-mode SFPs. You were too fast, I can't delete the thread. :-)

SX3008F fiberswitch, receiving signals from ER7412-M2 v1.0 2.5gig port via SFP ethernet adapter, ports working nicely with 1000Mbps SFP modules. Bought the new MC420L media converter, SM5110-SR...