Related to field in Task (Lookup field) disabled

Hi,

We are not able to to use the lookup field when we create a new task. The "related to" select box are disabled. I have read about the limit of 5 lookup fields, but i do not think it has anything to do with this, as this is a standard field in Zoho Crm.

This happens when we create a new task from within our leads tabs.

Please advice on how this field could be activated

Thanks in advance