This error is not generated by BSS, so you need not re-do your freq plans. LAC optimization/BTS reconfig would not solve this problem.
Try troubleshooting PCM trunks, probably mapping of the TSs. The best way to start is from the MSC end, I assume this problem is network wide rather than cell specific. Check whether some new recently added PCM. I would suggest start from A-inf then A-ter and then A-bis..This would reduce the time in troubleshooting.
Unfortunately my friend, there are no documents available for such errors, as they are network specific. All I have is good years of experience from which I can judge the situation.