... I will ask for support at Mochahost. They already solved a problem at installation where PHP module was missing ( PHP fileinfo ). May it be another PHP module is missing?

Whenever you login to your site, the PHP server environment is creating a so-called 'session cookie'. Normally, this cookie is kept in place as long as you're logged in in order to remember who you are and what actions you're allowed to do depending on your Ossn identity.

In your case this cookie is being removed for some reason - and getFriendRequests() fails because Ossn no longer knows which friends of whom it has to retrieve.

Consult your provider and ask him to solve this problem (losing sessions). Aside from that things like that are known to happen on cheap or free hosting because of limited resources. If this is the case you better change your provider.

Error log content is:
[16-Mar-2019 20:16:26 America/Chicago] Error: Call to a member function getFriendRequests() on boolean in / /* /* /* /components/OssnNotifications/ossn_com.php:109
Stack trace:

0 /* /* /* /* /libraries/ossn.lib.page.php(67): ossnnotificationpage(Array, 'notification')

1 /* /* /* /* /index.php(22): ossnloadpage('notification', Array)

2 {main}*