In 3.4.0 a validation of "period" fied of user Media was added.

Before, it was allowed to have

 1-7,00:00-24:00;

in 3.4.0 this field validation was added, disallowing trailing semicolon

 1-7,00:00-24:00

So the script to add user media could stop working after migrating to 3.4. There seems to be no mention of that in API changes

https://www.zabbix.com/documentation/3.4/manual/api/changes_3.4

or upgrade notes (if that is related at all). FYI, the server upgrade patch works as it should, removing the trailing semicolon.