One click upgrade is in 3 versions, for PS 1.5, 1.6 and 1.7.. Did u tried to install correct version?
I would recommend manual upgrade to 1.7.0.6.
barta.jakub: after unsuccessful upgrade to 1.7.0.6 (500 error after one click) I instantly upload all PS 1.7.0.6 files overriding all. Then PS start working, only Ajax request do not work..
Ináč ak tu sme nejaký Slováci, či Česi, nebude rýchlejší náš materinský jazyk?
Keby niečo, pošlite mi PM. Môžem sa na PrestaShop pozriet cez vzdialenu reláciu, TeamViewer..