The following reasons might explain this.
- The contact has disabled the InMail function
- You have already sent an InMail to this contact
- Your InMail credits are used up (wait until the next month, do receive more InMail credits)
In all the cases mentioned above, the InMail function is disabled.
Open the contact for which you see the "Profile has conversation disabled" message and try to send an InMail. This will show you the exact reason.