[SOLVED] [incomplete update] broken backend / Failed to start the session ... / CallbackController line 127

Hi there,

I get a problem on two websites since the lateste FC-update that prevents them to enter the backend.

1) user enters blue backend-login-screen:
/administrator/

User sees 6x echos
"Warning: count(): Parameter must be an array or an object that implements Countable in [...]/plugins/system/flexisystem/flexisystem.php on line 627"

and the login-form.

2) user logs in.

3) user sees broken backend with above 6 echos ("Warning: count(): [...]") near the broken / empty admin-menu.
and in the page-body a joomla-formatted Error:
"Failed to start the session because headers have already been sent by "[...]/libraries/src/Cache/Controller"/CallbackController.php" at line 127.
and a button "back to control center". this is the only option.

4) user presses above mentionend button.
-> user is now logged out (probably since no session was ever started.)

[Setup]
Setup is most likely (since i can not see it via backend):
J 3.9.4
FC 3.3.1.3

Hello

it looks like this file is some old version

there is no "count()" method at 627 line for 3.3.1.3

Solved, thank you!

I did a rollback, then installed latest FC via backend > install from web. (Somehow, the two installations did not offer the FC-update the regular way).