The issue was reproduced in-house with the following steps;
1-Create a custom (shared) page as a project administrator.
2-Share the page across several projects
3-As a subscription or workspace administrator remove the users access to one of the projects for which the page creator had access to.
4-The error then is presented 'Access Denied' when attempting to edit the page.
In the case of the customer the following scenario caused the error; some of the child project(s) which the custom page was originally shared to were re-organized and permissions were changed in the process. The offending project(s) were identified with the assistance of a subscription administrator, the project(s) were removed or unselected from the shared page and the issue was resolved.
To resolve the issue; restore permissions for the user in the offending project or remove the project from the shared custom page.