Not sure what is causing sql error 306 - sql-server

Question

I originally had the following query on sql:
SELECT PROJECT_TYPE.PROJECT_TYPE_NAME, PROJECT.PROJECT_NAME, STATUS_TYPE.STATUS_TYPE_NAME,
COUNT(*) AS Tasks
FROM TASK
INNER JOIN PROJECT ON PROJECT.PROJECT_ID = TASK.PROJECT_ID
AND PROJECT.POURCENTAGE < 100 /* the following ANDs filters out inactive projects */
AND PROJECT.PROJECT_TYPE < 6
AND PROJECT.PROJECT_STATUS_ID < 3
INNER JOIN PROJECT_TYPE ON PROJECT.PROJECT_TYPE = PROJECT_TYPE.PROJECT_TYPE_ID
INNER JOIN STATUS_TYPE ON TASK.STATUS_TYPE_ID = STATUS_TYPE.STATUS_TYPE_ID
INNER JOIN PRIORITY_TYPE ON TASK.PRIORITY_TYPE_ID = PRIORITY_TYPE.PRIORITY_TYPE_ID
AND NOT PRIORITY_TYPE.PRIORITY_TYPE_NAME = 'Hold'
AND STATUS_TYPE.STATUS_TYPE_NAME IN ('To do','In Progress', 'To Verify') /* only include projects in these 3 statuses */
GROUP BY PROJECT_TYPE.PROJECT_TYPE_NAME, PROJECT.PROJECT_NAME, STATUS_TYPE.STATUS_TYPE_NAME
order by 2, case status_type_name /* orders by status */
when 'To do' then 1
when 'In Progress' then 2
when 'To Verify' then 3
else 4
end
which correctly displayed the count of tasks in a certain status as such. However, I wanted to replace the
project.project_name
table with the shorter description listed in another table called
project.project_desc
, which looks like this. I attempted to implement this by replacing all project.project_name's with project.project_desc, and adding
AND PROJECT.PROJECT_DESC IS NOT NULL
under the first inner join. However, this results in the following error
SQL Error (306): The text, ntext, and image data types cannot be
compared or sorted, except when using IS NULL or LIKE operator.
I'm not sure what's wrong with this now, all of the null values should be filtered out so i'm confused on why the query doesn't run the same way as it did before, just reading a different table instead. I do not have access to edit the database, so these changes must be implemented through the query.

Answer 1

FWIW, I find this easier to read...
SELECT pt.project_type_name
, p.project_name
, st.status_type_name
, COUNT(*) tasks
FROM task t
JOIN project p
ON p.project_id = t.project_id
JOIN project_type pt
ON pt.project_type_id = p.project_type
JOIN status_type st
ON st.status_type_id = t.status_type_id
JOIN priority_type xt
ON xt.priority_type_id = task.priority_type_id
WHERE p.pourcentage < 100
AND p.project_type < 6
AND p.project_status_id < 3
AND xt.priority_type_name != 'hold'
AND st.status_type_name IN('to do','in progress', 'to verify')
GROUP
BY pt.project_type_name
, p.project_name
, st.status_type_name
ORDER
BY p.project_name
, FIELD(status_type_name,'to do','in progress','to verify')