Problems with the gradebook: Hiding/Unhiding categories broken

When trying to hide/unhide a category in the gradebook, I receive this error:

A required parameter (eid) was missing

However, the link followed by clicking the "Hide" option in the "total" row does include an "eid":

https://[...]/moodle/grade/edit/tree/action.php?id=7861&sesskey=bL7Cupqyzd&eid=ig55727&gpr_type=edit&gpr_plugin=tree&gpr_courseid=7861&action=hide

Yet, when displaying the error page "A required parameter (eid) was missing", the eid=... is in fact no longer present:

https://[...]/moodle/grade/edit/tree/action.php?id=7861&sesskey=bL7Cupqyzd&gpr_type=edit&gpr_plugin=tree&gpr_courseid=7861&action=hide

To reproduce:

• Add category to the gradebook of a course
• Try to hide it using the "hide" option in the "total" row.

Additional problem:
The traditional workaround seems to be available only to some [?] categories.

• Click "Edit settings" in the row displaying the category name.
• Then, open "Category total" and modify the "Hidden" checkbox.
  This works for freshly hidden categories. Categories hidden a few weeks ago (on the 4.1.1+ back then) just keep the "Hidden" checkbox checked even after saving it in unchecked state.