Plugin activation = Fatal error: Allowed memory size of...

I have a multisite subdomain network with about 40 sites and not many posts.

After adding a bunch of sites; I'm trying to activate a plugin both network and per site, and getting the memory allocation error...

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 46662537 bytes) in /wp-includes/wp-db.php on line 1087
which is
1086=// Log how the function was called
1087=$this->func_call = "\$db->query(\"$query\")";

I deactivated all network plugins and got the same error; after disabling/enabling other plugins the error changed to
wp-includes/wp-db.php on line 931
$error_str = sprintf( __( 'WordPress database error %1$s for query %2$s made by %3$s' ), $str, $this->last_query, $caller );

Fatal error: Allowed memory size of 268435456 bytes exhausted... if you look closely, that is 256MB... huh?!

I have VPS with good specs, php configuration memory_limit set at 64MB, each blog is allocated 100mb, I edited wp-config to add: define("WP_MEMORY_LIMIT","64M");

I can activate less complicated plugins (the one in question creates a number of db tables), so might be an issue with that particular plugin, but it was working fine before, and the memory allocation of 256MB is a puzzle. I'm wondering if something about limits to the database but I can't find anything about that.

I did a 'repair database' in cpanel and everything reported OK

I thought about increasing php memory limit, but if I set it at 64MB and I'm allocated 256MB - I have no idea what that means.

Any idea what might be going on? and how to maybe remedy it?