This can have multiple causes and is most likely setting related.
Please navigate to the planner and scheduler and perform the following checks:
- Is the scheduler set to "active"? If not, please toggle the switch.
- Has the quota end date passed? If this is the case, the project is not accessible anymore. Please check the quota end date and adjust if necessary. You may do this by choosing "edit" when clicking "activities". Adjust the end date to a date that lies in the future.
Note: the quota end date can not surpass the scheduler end date.
- Is the user added to the concerning quota row? If not, select the quota row and choose "edit" by clicking "activities". Search for the user and add him or her to the quota row
Save your changes.