ACF Pro: v5.6
There appears to be a bug between these specific versions of the plugins. The following issues have not been present before the update to ACF Pro v5.6.
With both plugins installed and activated, the symptoms of the bug include the inability to add new flexible content layout rows to either a new page or existing pages. When you click on the Add Row button and select a layout from the list nothing happens; no new set of fields appears.
If we deactivate EventOn, the flexible content with ACF Pro v5.6 work as expected.
If we leave EventOn activated and revert ACF Pro to v5.5.14, then the flexible content works as expected.
In the meantime we have reverted to ACF Pro v5.5.14 which does not appear to have this conflict.