Going back to the original question, on top of the specific pressures related to rest-day working:
there is a general shortage of qualified people in many industries, businesses and voluntary organisations (and in other European countries).
There are various reasons:
the safety precautions taken during the pandemic have delayed training/recruiting;
more people than expected have not returned to previous work post WFH, or have decided they can afford to retire/cut down;
there is a considerable amount of additional long-term sickness caused by covid or lockdown (take your pick!);
people are more likely to stay away from work if feeling unwell.
Not all these necessarily apply to traincrew, of course.
And it has happened before - back in the late 70s the Southern in particular was desperately short of guards, and cancellations were common, as was a lot of overtime.