ok so i'm using the operamini beta version (because i like it better than the plain operamini one)
since i've got nothing better to do all day; with the extreme mode data savings on, the text wrap and single column view off, i've managed to log in on tumblr
no special tricks, i just kept clicking and clicking that darn log in button, re-typing my password or e-mail when necessary, and just clicked and clicked no matter how many times it refreshed on me
after like, the 60th time doing this the browser pretty much just gave up on me and went, "okay, jesus, you know what? whatever. you win, go browse your tumblr with data savings on i don't care."
out of pure curiosity i tried it on the regular opera mini browser too and it worked just fine after 40+ struggles