I've had this problem for a long time, and only just solved it with 
the help of the hints on this ticket.



The suggestions given worked, but only when I dissected the upgrade 
script that was supposed to fix it; 
scripts/upgrades/1.1_to_2.0.mysql.sql



I checked out the mnemo_memos table structure, and the memo_category 
column was indeed still an integer, which was the cause of the 
problem, because the Horde_Categories backend changed that IIRC.



Anyway the upgrade script failed with:



ERROR 1091 (42000) at line 9: Can't DROP 'memo_modified'; check that 
column/key exists



Which of course resulted in the portions of the file which correct 
memo_category to a VARCHAR(80) were not being run.



Doing a C+P of the remainder of the file directly into a MySQL shell 
has fixed the issue.



I'll open up a separate bug about improvements to the upgrade scripts, 
because I have some ideas on that, and I think at heart this bug 
relates to the use of the upgrade scripts in the wrong order, or 
re-use of them.

No feedback.

I can't reproduce this.



Do categories work for you in any other applications? Did you upgrade 
from an earlier version of Mnemo? If so, are you sure you ran the 
upgrade scripts? Make sure the category field in your database is a 
varchar, not an integer.

If I associates a categorie with a note the column display always 
"unclassified".