Deleted every task?
Are you use you've got admin access (and thus see "actually all tasks") ? Otherwise, if you don't, there may well be tasks that you don't see.
If you have DB-access (or can get a DBA to run it), try the following:
select LDTC.*, LDT.TASK_NAME, LDT.CONSOLEUSER_IDN, CU.DisplayName as 'Task Owner' from LD_TASK_CONFIG LDTC (nolock) LEFT OUTER JOIN LD_TASK LDT (nolock) on LDT.LD_TASK_CONFIG_IDN = LDTC.LD_TASK_CONFIG_IDN LEFT OUTER JOIN CONSOLEUSER CU (nolock) on CU.ConsoleUser_Idn = LDT.CONSOLEUSER_IDN where LDTC.LD_SCHEDULE_EXE_IDN = 1
The output of which looks something like this... (click on the picture for full size image).
You can filter further (such as on the agent config name) if you want, but I figured let's start with "seeing everything".
It's POSSIBLE (though not very common / probable) that there's some sort of logical DB corruption going on and you've got orphaned data.
*IF* that's the case, I've documented how to find it & handle it in here (that *WILL* require SQL Profiler & DB-access, so you may need to involve your DBA) -- How to find and deal with orphaned data / What to do when data re-appears after being deleted .
Should help figure out what's happening.
If you're at a complete loss, you can get in touch with support who can have a look (at a backup of) your database. That will require you to have a DB backup & send it to them (we have a process for that).
Hope that helps.
Is that Agent being used in any other place like provisioning?