Sharepoint workflow task can't be completed

after days of trial and error i hope you can guide me to the right direction.
I create a complex solution in Sharepoint 2013 with 2010 Worflows to organize absences of our employees.

Each employee must fill out a proposal form for each type of absence. The proposal can only be accessed by the creator, employee and supervisor(s), let me call them editors.

The supervisor approve or reject this proposal and if approved it would be copied to another list that shows the absences in a diagramm (gantt). This absences list element must be seen for other employees.

My idea now was to create a field in the approval workflow task for the supervisor to define wich employees should see this absence.

I create a task formfield (new not a site column) with type person or group, set the multiple option and want to save the value (employee accounts, let me call them reader) back to the proposal element. After approve of all supervisors the proposal will be copied to the absences list (but not with all data) and the access will be given for each editor and the defined reader.

My problem is if i want complete the task (Approve-Button) then after 10 seconds the "wait for complete" page close, the task status is not started and i can’t edit the task because of the error

Access denied, workflow in progress. In the logs is nothing to find.

So i think it isn’t an error but a infinity search of information or something else that let the workflow hang.
I have tried very much, one of this is that there is a bug while using site columns instead of custom column for task form field (see Updating list from custom task form field? ).

I also tried to use a single person or group field. This works but doesn’t match the requirement.
Did someone have such a related issue and a solution? Did i make a mistake within my thinking process?

Has anyone another solution to solve this requirement?
I hope my english is not so bad and you understand what i want 🙂
Thanks a lot for your help.