Please give us result of following queries :

SELECT w.id as workId, w.idResource as idResource, w.work as work FROM work w WHERE w.refType='Activity' and w.refId=659;
 
SELECT w.id as workId, w.idResource as idResource, w.work as work, a.id as idAssignment FROM work w, assignment a WHERE w.idAssignment=a.id and a.refType='Activity' and a.refId=659;

Hello Babynus,

After the execution of the first SQL request, here is what PhPMyAdmin return

After the second SQL request :

One precision

• My tables begin all with prefix prj, that why I modified the SQL request to take this point into account.

You have issue with 6 lines of work returned by second query : they are linked to assignments referencing the activity, but not to the activity itself.

Please give result for :

select * from prjwork where id in (32,33,34,35,50,94);

It will return the corresponding lines.
Go to corresponding week / resource on Real Work allocation screen and remove work.
You'll then be able to delete assignments.

Hello

I executed the SQL request

SELECT * FROM prjwork WHERE id IN (32,33,34,35,50,94);

and have this result

For the ressource 92 on the project 98, I go to corresponding week / resource on Real Work allocation screen to remove work but there is no work entered

For the ressource 15 on the project 95, I go to corresponding weeks / resource on Real Work allocation screen to remove work but there is no work entered

For the week 49

For the week 50

For the week 51

I can't delete real workload because I don't have anything in the field mentionned in the request 🙁

It is very weird as work lines do not reference the activities you were dealing with.
First 4 lines reference activity 270 but you were dealing with activities 658 and 659.
Moreover, you access real work for reasource 15, called Damien, and there were no Danien in Assignments for Activities #658 and 659.
I cannot understand, and I'm afraid I won't be able to help without a version of database to investigate. 🙁