I've had exactly the same problem but it seems to vary, on the event, sometimes, I get up to 1400, refresh and then impossible to get further than 1200, refresh again and get to 1300 with no problem. The first time it happened is when I got to lvl 6000. Even after 1 minute 20 seconds, I had done no damage and then, I was taken back to lvl 5995 and it took something like 10 minutes to get to 5996 (whereas I got to 6000 pretty normally) usually 3-4 secs per lvls up to there.
I've been having a browser open since the start of the event and I'm hardstuck at lvl 1400 ish and with this weird bug, I'm starting to doubt the possibility of completing it.