Problem Details:

Version used Sage CRM 2019 R1

Tasks & Appointments not synchronizing

Solution or workaround

There were two things we found:  In IIS Sdata application was giving 404 errors, we resolved the errors by 
1. The forms authentication was enabled, disabled it
2. Windows authentication was enabled on the SDATA application, Disabling Windows authentication resolved the issue
The sync engine is now updating and syncing with Exchange and Mailchimp.