why is excel changing a formula in a locked and protected cell

PBM is play by mail - one cell changes so for example i might have the following formula in e1 "= A1+B1 +C1" - so the sum of three cells are added together - not 3 cells side by side as shown but just used as an example here but when the file has been transferred to this other person the formula in e1 suddenly reads "=a1+b2+c1" but because  b2 is NOT a number cell but a text cell the result gives an error so it then disrupts further calcs that will need to read cells e1 answer.  but this only happens to one person, all the others the formula stays correct. it is not restricted to the same cell, the next time it happens could be in cell Z5 rather than E1. so it is just not making sense