I've seen a lot of people with this issue when trying to create a custom indexer but my issue is on a brand new install V22.214.171.124 with sample data.
When attempting to save any settings in the admin panel OR when viewing the index management screen i get:
There has been an error processing your request
Indexer model should extend Mage_Index_Model_Indexer_Abstract.
I've tried clearing the cache & checked permissions but all seems to be fine, frontend of the store is working correctly.
Any ideas why the indexer isn't working?