I have 2 installations in the same url and different paths. One for testing (with own database table) and one for "usage" (with own database table).
Both installations should contain the same files

I say on purpose "should"!
Some minute ago I was purging the caches (image/rss/html/search) of both installations.
The "test" version works still fine.
While the "usage" version shows a blank page.
error.log just says: 500
debug.log shows:
USER ERROR: MySQLi Error: ( SELECT DISTINCT `id` FROM `[prefix]images` WHERE `show` = 0 OR `albumid` in () ) failed. MySQLi returned the error You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 1 in /var/www/my_zpc/com/zp-core/functions-db-MySQLi.php on line 78
trigger_error called from query (functions-db-MySQLi.php [78])
from getNotViewableImages (functions.php [1568])
from getTagCountByAccess (functions.php [1191])
from getAllTagsUnique (functions.php [1116])
from tagSuggestJS (tag_suggest.php [64])
from tagSuggestJS_frontend (tag_suggest.php [50])
from call_user_func_array (unknown)
from zp_apply_filter (functions-filter.php [149])
from include (inc-header.php [37])
from include (index.php [1])
from include (index.php [130])
from index.php [31]
Then I overwrote functions-db-MySQLi.php by copy and pasting it from the test install. But no change.
But the "usage" installation can be accessed with my working browser where I am working as ZenPhoto-Admin and the page works all normal.
Any idea because this is really strange to me.