Is it happening on specific issue?

No on every issue i edit on issues.joomla.org

Hmm... I am trying to edit & save issues and do not have any errors. What do you change on edit?

In the most times labels status and ore milestones. Did you try your local or the live site?

On the live, but I do not change anything :) Now I tried on jtests tracker - and see this error too. Was able to replicate it locally. It is happening when you try to assign milestone.

8 Whoops\Exception\ErrorException

Object of class stdClass could not be converted to int

exception: Whoops\Exception\ErrorException: Object of class stdClass could not be converted to int in ROOT/src/App/Tracker/Controller/Issue/Save.php:366
Stack trace:
#0 ROOT/src/App/Tracker/Controller/Issue/Save.php(366): Whoops\Run->handleError(8, 'Object of class...', 'ROOT/src/Ap...', 366, Array)
#1 ROOT/src/App/Tracker/Controller/Issue/Save.php(155): App\Tracker\Controller\Issue\Save->updateGitHub(951, Array, 'closed', 'closed', NULL, 1, Array)
#2 ROOT/src/JTracker/Application.php(174): App\Tracker\Controller\Issue\Save->execute()
#3 ROOT/vendor/joomla/application/src/AbstractWebApplication.php(134): JTracker\Application->doExecute()
#4 ROOT/www/index.php(106): Joomla\Application\AbstractWebApplication->execute()
#5 ROOT/www/index.php(133): {closure}()
#6 {main}