What matters to Linux operating systems (to their kernel thru system calls done by every programs) is not user names but user ids or uid-s. It is a small positive number (not a name), like 1234. Read credentials(7).
You need to ensure that on both your Debian and your Ubuntu the
daniel user has the same uid. The
daniel name is not very important (and you might want, but need not, to use a different name). Probably Debian and Ubuntu are using different uid-s/gid-s for
daniel. Read also id(1) and use
id -a. And use the stat(1) command on your home directory, so
stat /home/daniel to understand what uid and gid it has...
I assume you can get superuser power on both OSes, e.g. by running
sudo. Otherwise you need to boot from an external media in rescue mode, or boot your kernel with
The mapping between user names and their uid-s is often done in
/etc/passwd, so read carefully passwd(5) (today that
/etc/passwd does not contain any password information anymore, but that file path did not change for historical reasons. See also shadow(5)). BTW, that mapping could -with care- be done otherwise (but usually is not) and how it is done is configured in
/etc/nsswitch.conf (which I assume you did not change, see nsswitch.conf(5) for more)
Likewise the mapping between group names and their gid-s is done in
/etc/group, so read carefully group(5).
So, first run the commands (not GUI programs, in your case they are confusing you) to understand what uid/gid is used for
daniel on both Debian and Ubuntu. You probably want to run the
grep daniel /etc/passwd command (see grep(1)). Likewise, run the relevant command to find your gid (on both systems).
Then decide on a common uid and gid for both systems (Debian and Ubuntu). It should be unused, and you need it to be the same (small positive number) on Debian and Ubuntu. I recommend choosing a number above 4000 but below 65000 as your common uid (and likewise for your common gid).
Change the uid and gid by editing -as root- carefully with an editor (like
nano, etc) the
/etc/group files on both systems to share the same uid/gid on Debian and Ubuntu for your user. Use sync(1) after that edition (it probably is useless, but you want to be sure that your edition goes to the disk).
(if you mess up that step, you could have some unusable system)
Then (as root) change (once) recursively the ownership of your home directory, using chown(1) command. You probably want to run something similar to
chown -R daniel:daniel /home/daniel (the first
daniel is a user name, the second is a group name, the third is a directory entry). After thatc consider using chmod(1) to change the permission (probably as
chmod -R a-rx /home/daniel), since you should not have world-writable home or own directories (this is a huge vulnerability).
NB: prefer, for administrative tasks, to use commands not GUI