The buffer setting only applies to port logging which is enabled under the Advanced Serial settings >
__Enable Port Logging
If this value is not check marked, the log size would have no bearing here.
With regards to the problem, what exactly is the behavior you are seeing? Had this been working for some time?
Some things to look at by running the following command from the root prompt:
#> info serial:1
Values showing for norun, noflow, nframe, nparity, and nbreak are indications of errors.
You also might want to check on possible network latency/bandwidth issues.