Ignacio,
it is exactly as you say: the upgrade goes as planned and is successful (the upgrade process itself), I login with my account and get a white screen (see attachments).
I tried and described three different alternatives to try to get around this blocking issue, all three alternatives are described step by step and accompanied by log files. Unfortunately none of these alternatives succeeds in making the upgrade from 1.5.3 to 1.6.1.
I'll stick with version 1.5.3 since there is no way for me to upgrade.