I've added evoc to an existing drupal 6 site, and now see following at login:

user warning: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '=1 AND n.status = 1' at line 1 query: SELECT DISTINCT(n.nid) FROM tatoo_node n LEFT JOIN tatoo_content_field_semtag f ON n.vid = f.vid WHERE f.=1 AND n.status = 1 in /data1/tatooweb-d6/sites/all/modules/evoc/modules/evocreference/evocreference.module on line 689.

It's repeated some dosen of times, does not seem to have any really bad effects. The problem is in "f.=1" part. Besides, the content_field_semtag is quite empty, not sure if this matters:

mysql> select * from tatoo_content_field_semtag;
+-----+-----+-------+------------------------+
| vid | nid | delta | field_semtag_evoc_term |
+-----+-----+-------+------------------------+
| 60 | 55 | 0 | NULL |
+-----+-----+-------+------------------------+
1 row in set (0.00 sec)