BuddyPress.org

I’ve got a problem after updating my Buddypress.
I deactivated BP plugin.
Then uploaded latest WP and BP versions.
Then updated WP. It went smoothly.
Then tried to activate BP and got the error:

(Site) is currently unable to handle this request.
HTTP ERROR 500

I deactivated ALL plugins (deleted or renamed files via ftp) and use standard WP theme named Fourteen.
The site works fine but when I try to reach Admin Panel, I get HTTP ERROR 500.

I changes line

define(‘WP_DEBUG’, false);

to

define(‘WP_DEBUG’, true);

and then got the following error:

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 86 bytes) in /path-to-domain/wp-includes/wp-db.php on line 1837

What can be the problem?

WP v. 4.6.1
BP v. 2.6.2
I didn’t check what the previous version of BP was. Can it be the issue? Should I have installed all updated step by step?
No plugins.
No changes in any files.
No changes in WP theme Fourteen.
Everything worked fine before I reactiveted BP plugin.

I don’t think server, nginx, etc. is a problem because everything worked fine and the site continues to work fine except for the Admin Panel which doesn’t work at all.
When I deactivate BP plugin, the admin panel works fine as well.

Does anyone have any ideas on what can be done.