Problem

When saving an animation, using a solution based shading mode (Ray
Trace, Particle Trace, Radiosity), getting message "Error: 901 Saving
Sequence".

Reason:
The option menu for rendering method is set to "Use solution computed from frame:" but no solution was computed or the stored solution is incomplete.

Solution

Set the option menu to "Create single solution from frame", to create the solution needed. If the solution does complete, the setting "Use solution computed from frame:" when then function as expected.