This issue was posted in Microsoft TechNet Forum to which I responded and helped to resolve it.
Basically one of the users was getting Access Required error for Project Site Task List. The user was part of Project Manager group and was having sufficient permissions. Issue was occurring for only one Project Site and user was able to access the task list for other Project Site. I suspected, some issue related to Project Site Sync so requested to unlink and then relink the Project Site using below steps. This resolved the issue.
- Server Setting >> Connected SharePoint Sites.
- Select the row for the site you are having issue,
- Click on edit site address
- Make a note of “Web application” and “Site URL”. You need to re-use this information later while re-linking the project site again. Copy the Site URL and save it in notepad for later use.
- Select “Unlink the SharePoint site from the Project”
- Click on OK. You will get the message “Are you sure you want to remove the link for this project site. Click on OK.
- Again select the same row.
- Click on Edit Site Address .
- Make sure “Type a new SharePoint Site URL” is selected and enter the Site URL you noted in the Step 4 for “Site URL”. Make sure the “Destination URL” reflects the original site URL.
- Click on OK
Let us know if you have any additional questions.