[SOLVED] Strange performance issues

Hey guys, just a little update:
I haven’t gotten around to finding a solution yet, as I’ve been working out of the studio a lot lately.
But yesterday a strange thing happened:
I hooked my Z800 up to the internet (it’s usually strictly offline) to do some updates, and started mixing a project.
I was at it for three hours or so, without a single glitch. Then I remembered the network cable to the Internet router was still plugged in. I disconnnected it again and, lo and behold, the performance issues were back instantly.
So my Nuendo only behaves that way when it’s NOT connnected to the internet.
It appears to be something network related, but I can’t imagine what.
Another mystery: I’ve also got Cubase 8.5 running on the same computer, and there are no issues there, although I’ve been doing stuff in it lately that’s much more processor- and disk load-heavy.
Weird…