Error while adding Database Server to index

Thanks for reporting, and sorry it took a while for me to get back to you.
To me, this looks more like a problem in Drupal's PostgreSQL driver. It seems if you abort a transaction, the registry cannot look up classes anymore – which would be a bug in that system, I'd say.
However, I'm not even sure how this would be triggered in this case – it doesn't even seem like we have any transaction/rollback in the code, and all the code that would have them doesn't use the SearchApiException class, as far as I can see.
Anyways, I'm for now moving this to the Drupal Core issue queue for feedback.

I exported a search server with Features and tried assigning this server to the default node index. I am getting the same error message:

PDOException: SQLSTATE[25P02]: In failed sql transaction: 7 ERROR: current transaction is aborted, commands ignored until end of transaction block: SELECT r.filename AS filename FROM {registry} r WHERE (r.name ILIKE :db_condition_placeholder_0) AND (r.type = :db_condition_placeholder_1) ; Array ( [:db_condition_placeholder_0] => SearchApiException [:db_condition_placeholder_1] => trait ) in _registry_check_code() (line 3483 of /var/www/html/includes/bootstrap.inc).

I am using seach_api_db with PostgreSQL.

@drunken monkey - Any guidelines you can provide, much appreciated.

No, sorry, I'm pretty clueless here. It also doesn't seem at all obvious how to even trigger that error.
Maybe when te database structure is changed (creating/altering/dropping tables) during a transaction? Does that maybe cause problems in Postgres?
I know it caused problems in MySQL, though I'm pretty sure it got fixed there already. Unfortunate that we don't have any Postgres test coverage in D7.

Probably related to this: #2487269: Postgres insert queries that fail in a transaction break the entire transaction

This is a duplicate of: #3384672: [D7] PostgreSQL queries that fail in a transaction break the entire transaction