crons not working

[NeXuM]

This has been annoying me since years. Crons are not working on my forum (vb 3.6.4) i don't remember if they ever worked but it doesn't since many years.

I have to login in admincp and manually launch the crons jobs if i want my users to be promoted to the correct groups (i have a promotion set to promote users to a different group when they reach 150 posts).
It works fine when i launch them manually from admincp but it doesn't work when i set it to be launched automatically once every X minutes.

I am 100% sure that everything is correctly setup in the ACP crons section.

I have also tried to reset my footer template to default to ensure the function to launch crons is still there, but it doesn't change anything.

Any clue where i should look to fix this bug ?

Another "solution" would be to write my own PHP code to launch the cron jobs and include this code in the footer.
I have tried copying "admincp/cron.php" and just keeping the "runcron" function then including the page inside an hidden iframe to trigger it every time a user visit the forum. But it will only works if the user is a logged-on admin.