Maybe there is some shape with lock on text edit. You can remove that protection:
shape.get_CellsSRC((short)VisSectionIndices.visSectionObject,
(short)VisRowIndices.visRowLock,
(short)VisCellIndices.visLockTextEdit).FormulaU = "0"; //to remove protection
And it is better to "remember" old value and set it back after yours edits.