Can't login to Webfig 6.1 with Chrome browser after upgrade to 6.1: ERROR: Internal Server Error
(can login with Firefox at the same PC, can login with Winbox)

Give us detailed information about Chrome version, and Firefox version, as well as your OS version. Thanks!

Chrome Version 27.0.1453.93 (the same as it was 5 min ago when version 6.0 run WebFig without any errors)
Firfox 21.0, Firefox for openSUSE (run WebFig 6.1 OK)

Linux openSUSE 12.3 (Linux linux-ks1l.site 3.7.10-1.4-desktop #1 SMP PREEMPT Fri Apr 19 12:06:34 UTC 2013 (8ef74f8) x86_64 x86_64 x86_64 GNU/Linux)

Firfox 21.0 (Firefox for openSUSE)

I had to kill the Chrome task to log in after upgrade. Then it worked again.

Can't login to Webfig 6.1 with Chrome browser after upgrade to 6.1: ERROR: Internal Server Error
(can login with Firefox at the same PC, can login with Winbox)

Still, the same for me. I've just updated my RB750GL to 6.1.
chromium-browser-28.0.1500.52 (canot login)
iceweasel-22.0 (can login)

Clear Cache in browser and in proxy if you have one.

Same issue with Chrome 28 (Win 7). Cleared cache, cookies, restarted. Chrome doesn't seem to like 6.1.

Can get in with IE10 just fine.