Just for share. I installed a local HTTP proxy (Privoxy) on my laptop recently. Today I've noticed that I can't login to the MikroTik web interface:

Authentication failed: invalid username or password.

Spent two hours to investigate, changed password, updated RouterOS and RB f/w and so on. BTW, winbox works perfectly.

It turns out that I just had to turn off the proxy. That's all, login works as expected. Re-login also works, even through the proxy.
Thanks for your attention, maybe this will help someone.

P.S. RB951G-2HnD