Hi,
After updating NCS folder to latest Master branch, SES4.52 is not able enter into debug mode.
Receiving error as: failed to set startup compilation break point: main symbol not found
Work around given in below case, helped me to go into debug mode.
https://devzone.nordicsemi.com/f/nordic-q-a/57342/suddenly-can-t-debug
I think NCS in master branch has some bug which preventing it to enter into debug mode.