If you receive a “500 server” error when you attempt to download your calendar’s ICS file, there are the possible reaons and solutions:

Solution 1:
I found that the Slider WD Plug-In conflicted with the option to download an ics file from your timely calendar.  Disabling this plugin solved the problem.

Solution 2:
It seems we were having this issue with PHP version 7.1 (7.0 seems to work).

Our developer commented out the following lines and that seems to have eliminated the error message and all events synced successfully:

plugins/all-in-one-event-calendar/lib/iCal/iCalcreator-2.20/iCalcreator.class.php” and the lines are 8003-8006

if( $startdatets <= $wdatets ) { // only output within period
$result[$wdatets] = TRUE;
// echo “recur “.date(‘Y-m-d H:i:s’,$wdatets).”<br>\n”;//test