Why doesn’t my sed command work properly? It replaces old values with errors

The sed command that I use is:

sed -i 's,oldvalue,newvalue,g' *.txt

When I use the above sed command each with the below examples (1, 2 and 3), the results contain errors.

Example 1

Old value:

AEP/e2cy//rSq,W8VJuTMb1vGeeyxcr

New value:

/uK2S9v6A\NnW,PbRB+WUwn/Uu83,Gym

Example 2

Old value

8000::/1

New value

128.0.0.0/1

Example 3

Old value

10.10.77.96/32, fd00:10:10::c18d/128

New value

2001:db8:abcd::/48