Please get QCCR2A82175 from Micro Focus support to resolve this issue.
Schedules with exclude option not supported in dp 10.20 after migration from 9.0
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I eventualy found a post considering the same issue and applied the fix described there (https://community.softwaregrp.com/t5/Data-Protector-User-Discussions/Issue-with-Backup-Schedule-after-migration-from-9-08-to-10-20/m-p/1680982/highlight/true#M158876)
All seems to be working now.