FYI, actually the matter looks somewhat different now (but still probably has nothing to do with Syncthing, at least I hope so).
Most computer security companies seem to have agreed that any access to that website shown in the table above (which has ceased to exist a long time ago) is indeed a sign of malware infection.
Therefore, Internet providers have joined forces internationally, and automatically redirect this website (among many others) to a so-called sinkhole address. If an Internet provider then registers traffic on this sinkhole address, the authorities of the geographic region evident from the source IP address are contacted, who in turn can then issue warnings or initiate police investigations, as the case may be.
Thus, although I have not yet been able to investigate the PC in question, it seems that it was indeed infected by that malware.
Also, it is still not apparent to me how the PC could have been infected in the first place. It had been running automatically in regular intervals for a while, however only for file synchronization via Remote Desktop and Syncthing, and without any physical user interaction on-site.