Thank you for your help! Encouraged by point 3 and after finding out that from SFOS 4.4 some hybrid-recovery.img files only produce a boot loop ([Xperia 10 II] Unable to reach Recovery Mode - #11 by launchpad), I tried the latest one from 4.5 which brought me into recovery mode. I could copy all important files and even extract the hybris-boot.img and boot again into the intact UI.
2 Likes