We have a CiviCRM 5.15.0 installation under WordPress 4.9.10 and 2 MySQL db: one for Wordpress data and one for CiviCRM data. We manage more than 6500 contacts.
Going inside a contact we have this error message with a blank popup adding a meeting or phone call in the contact summary:
Network Error Unable to reach the server. Please refresh this page in your browser and try again.
There are NO ERRORS adding contributions, registering for events, adding pledges, sending emails.
Analyzing logs we have no errors on Apache, but we found a
You can see how the errors come out on this video https://youtu.be/lrMq6A2t9Uk