Yeah, I just got that too. It says dst, so it's obviously something to do with Daylight Savings (UK switched to BST today).
I'm getting it randomly whilst browsing the forums, and when logging in now. Seems to happen every time I try to open a different thread after browsing in another tab for a few mins.
Its been happening to me since the start of the week. Anytime I log in I get the invalid URL error and I've got it a few others times outside of that but I forget exactly what I was doing.
I've started getting this since the weekend whenever i follow a link or try to open one in a new tab (already logged in). Hadn't previously had a problem.
Looks like anytime you load a forum page (regardless of if it's logging in, loading a topic, trying to preview or send a post or PM, etc) after spending more than a certain time without loading a page (e.g. on first login, but also if you've had a written post open for some time before posting) you will get the error page.
For the one that has =dst at the end, as this is a vBulletin thing caused by the UK switching to BST I've just set my profile to have daylight savings always on instead of automatic for now and it fixes this particular invalid redirect.