Hello Adrian,
so unfortunately it has still no clear error.
In your .xsession-errors is tcosmonitor mentioned.
As this package got 2016 removed [1] from debian - do you
have still installed tcosmonitor or tcosmonitor-common?
If yes, maybe that might cause some troubles?
If no, which file tries to load "tcosmonitor.shared"?
Do you have other packages not contained in testing and
any other sources.list, that may cause trouble?
LANG=C apt-show-versions | grep No
Back in 2010, was this system installed from a
regular debian installer?
Kind regards,
Bernhard
[1] https://tracker.debian.org/pkg/tcosmonitor
