Have you run a full backup on the primary since LS broke?

Use this query against the primary and then against the secondary to find out if you have

select name, differential_base_lsn from sys.master_files

where database_id = db_id('yourdb') and type = 0

If the differential base lsn matches on both primary and secondary, stop all LS jobs.

Take a differential backup on the primary and then restore this to the secondary. Now restart all the LS jobs.