Sugar 12 Leads conversion problem

Hi,

after upgrading to onpremises Enterprise v12, one of my users is experiencing an issue in converting leads.
When clicking the Convert button, nothing shows. If he tries to move to another page, he is warned he didn't save the changes, then i guess something started in the background.
If i grant him Administration role, conversion works normally.
His permissions didn't change after the upgrade

What should i grant him to allow conversion?

Thanks a lot