Steps to reproduce:

1) Create a group called "Test" through Administration->Manage Groups
2) Go to Browse Space->Space Admin->Permissions
3) Edit Group Permissions
4) Manually type "test" in the "Grant Permission to" section (Please do not use the Group Picker in this step) and add this group to space permissions
5) Save All

The group that is added to space permissions is "test" instead of the expected "Test" group name. Hence all users belonging to "Test" group, when try to access this space get a "Not Permitted" error as "Test" and "test" are now considered as two different groups.

Workaround: Removing the faulty group name and then using the "Group Picker" for selecting the group instead, will resolve the problem.