Can I deactivate log shipping jobs without raising errors?
This normally occurs when the log-shipping is not removed cleanly. To remove the orphan entries you have to delete them from msdb logshipping tables.

On Primary :
delete from msdb..log_shipping_monitor_secondary
where secondary_database = '' -- secondary db name goes here 
delete from dbo.log_shipping_secondary
where primary_database='' -- primary db name goes here 

On Secondary:

delete from msdb..log_shipping_monitor_primary
where primary_database='' -- primary db name goes here 

delete from dbo.log_shipping_primary_secondaries
where secondary_database='' -- secondary db name goes here