One specific user is unable o add an Other Work instance to the Modern UX timehseet.
He has been able to enter time against that instance up to this week. But this last week is unable to do so.
It turns that the Other Work instance was present on the prtimeentry table, but not visible on the Modern UX timesheet as a selected time entry.
Checked the timesheet instance on the Classic UI.
The time entry was visible on the Classic UI timesheet.
Removed the time entry form the Classic UI.
The time entry was selectable on the Modern UX afterwards