"View other users' issues (if not set, access will be limited to issues reported, handled, or monitored by the user)" - does't help

See also 0033342 that might be the same issue.

Did you check on page "Manage" > "Configuration" > "Workflow Threshold" of the affected project if "Monitor an issue" is set for the affected access level?

You do not provide much context or detailed information...

I did not actually test to confirm, but the problem may have been introduced by the fix for 0029454. Make sure that the logged-in user who is attempting to add others to the monitoring list, has an access level >= monitor_add_others_bug_threshold

Yes, it was working till 2.26 update :[

obraz.png (20,502 bytes)   

obraz.png (20,502 bytes)   

You dont have set View other users' issues (if not set, access will be limited to issues reported, handled, or monitored by the user) for reporter access level.
I guess you are trying to add a user without that right to the monitoring list.

You dont have set View other users' issues

@atrol that's it.

I just tested with this configuration, and indeed in 2.25 a user X having monitor_add_others_bug_threshold was allowed to add a user Y having monitor_bug_threshold to an issue that Y had no access to because of limit_view_unless_threshold, which effectively gave Y visibility on the issue.

in 2.26 with the same configuration, it is no longer possible to do that so we do have a regression because it was in my opinion intentional to allow adding a monitor in order to grant them access to the issue.

I don't want that every reporter have access and view to all project submissions, I only want them to have a view as an observer for only one ticket.
I checked only the first check (=only the monitor can see the submission), but it doesn't work (error #31).

After analysis, this appears to be a duplicate of 0033404