The problem is in helper_get_current_project() from helper_api.php, if user can access only 1 project, function shouldn't return ALL_PROJECTS.

I have modified the file and it is works, edit helper_api.php line 209 in the if condition.
if( !project_exists( $t_project_id ) || ( 0 == project_get_field( $t_project_id, 'enabled' ) ) || !access_has_project_level( VIEWER, $t_project_id ) ) {
$t_projects = project_cache_all();
$t_projects_count = 0;
$t_last_id_project = ALL_PROJECTS;
foreach($t_projects as $tmp_project_id => $tmp_project){
if( access_has_project_level(VIEWER, $tmp_project['id'] ) ) {
$t_projects_count++;
$t_last_id_project = $tmp_project['id'];
}
}
if($t_projects_count == 1) {
$t_project_id = (int)$t_last_id_project;
}
else{
$t_project_id = ALL_PROJECTS;
}
}
I think a question is the function should return only the project id or the project list (with "all projects" and the project) should be displayed ?

This issue is not reproducible with the current version of MantisBT.

We recommend that you upgrade to the latest stable version [1]; if after doing so the problem persists, do not hesitate to reopen the issue, with additional relevant information.

[1] https://sourceforge.net/projects/mantisbt/files/mantis-stable/