too long fene bond in the boundary condition

I am thankful for your answer.
What is happening here is this happens at the boundaries and it is just weird. My forces are stronger in the middle of the box in comparison to the boundarie. I make my box bigger and I observe that fene potential error appears again when my polymer reaches to the boundary…

Nothing weird or ironic about it. The FENE force diverges towards infinity as the bond-length approaches the maximum-allowable value, hence the name finite extensible nonlinear elastic (FENE). Larger forces require a smaller timestep in order to preserve numerical stability. This is similar to the problem of intense headaches caused by banging your head against a wall. The solution is to stop, or at least banging your head less violently. In your case, that would correspond to reducing the strength of the fixes that you are using to drive the system.