not quite, as I said I have reached 1000mbit with this setup previously. I solved the issue by trying with a short patch cable I have lying around and got 1000mbit. This meant it had to be the cable, or maybe connectors that got worn out. So I went back to the old cable and held the connector really tight by applying force to it and, behold, the card negotiated Gigabit speed.
I never had this kind of performance degradation happen before, and I didn’t know it is a thing. I guess one keeps on learning new things. It also says something about the robustness of the Ethernet protocol: a poorly performing cable would cause the link to gracefully degrade to the lower standard instead of downright not working or corrupting data.