hmmm.. have you tryed to login in minibb and then to reach /wp-admin?

Yes, this scenario also worked.

on my test page wp 5.1.4 (highest WP ver. that is possible, becouse my hosting runs 5.4.45) synchronising with minibb 3.4.2

need to upgrade WP to higher and hope it will work on epolemika.ebaznica.lv

cheched more... unfortunetly the found issue

if i am login from miniBB and go to wp-admin it shows that i am not logged in in both minibb and wp
but when i login throught wp and check-box "remember me" everything works fine

This still indicates that there might be some wrong cookies paths configuration on your server. You have to check these paths and you have to check/debug what cookies are set.

I have tested it all without checking this box.

what changes you made in /wp-includes/default-constants.php ? all, some, one or none?

i have updated WP to 5.4. and miniBB is running 3.4.2
php now is 5.6

If you mean if I've edited this file during the installation, then the answer is: I did not modify this file at all. I've installed WordPress just following their standard installation guide, i.e. put the database information using the form fields during installation, and put the initial admin info via the similar form. I didn't went into modifying their file. It was just a pure clean installation from scratch.

May be you could try the same. In some sub-folder, re-install Wordpress and install the newer version of miniBB, following standard README guides. If it works, then you could switch to the original database. At least, you must make sure the standard installations work on your server, without trying to upgrade a customized version.

Thank you Paul for help and suggestions, and sorry for bothering..

it seems that I've solved problem.. rhere was http / https issue (not cookie)
WP was on https, but minibb on http
now both on https... and all login logout works

epolemika
Thanks for report! Great you solved it!
Yes, http/https could be most of the common issues :-)

that pretty good paul, thx