Slaves show different results for query

Hi,

I have a strange case where a SELECT for a primary key returns 0 rows on one slave, while it returns the correct 1 row on another slave and on the master. It does however return that row on all slaves when queried with LIKE and trailing or leading wildcard.

psql version is 9.3.5, because that's what comes with Ubuntu 14.04 LTS
Master runs Ubuntu 14.04 LTS
Slave 1 runs also Ubuntu 14.04 LTS
Slave 2 runs Mac OS X 10.7, pgsql installed via homebrew

Both slaves are configured with streaming replication, and I've been using that setup for years, starting with psql 9.1, with no problems so far. Suspecting some weird problem, I already re-initialized slave 2 with a fresh backup and started replication from the beginning, so the database is fresh from a master copy, and is verified to be current.

2015-05-19 20:53:43.937 CEST LOG: entering standby mode
2015-05-19 20:53:43.974 CEST LOG: redo starts at 31/3F000028
2015-05-19 20:53:45.522 CEST LOG: consistent recovery state reached at 31/40CCE6E8
2015-05-19 20:53:45.523 CEST LOG: database system is ready to accept read only connections
2015-05-19 20:53:45.604 CEST LOG: started streaming WAL from primary at 31/41000000 on timeline 1

So here's the query.

SELECT * FROM MyTable WHERE email = 'foo(at)example(dot)com';

This returns 1 row on master and slave 1, but 0 on slave 2, while this query:

SELECT * FROM MyTable WHERE email LIKE 'foo(at)example(dot)com%';

or this one

SELECT * FROM MyTable WHERE email LIKE '%foo(at)example(dot)com';

returns the correct 1 row on all three systems. Note that this works with the wildcard on either end, or also somewhere in the middle, doesn't matter. Note: "email" is the primary key on this table.

This behaviour is the same with any address to be queried, and is also the same on a similar second table. This does NOT occur on any other table, which all have integer primary keys. There is also no problem when I select for other attributes on these tables.

Does anyone have a hint?

Thanks
Maik