Typically you can ignore this error when it happens less often than once per minute.
In many cases it caused by Linux scheduler when it allocates time for another task.
When it happens every few second then it’s sign that FastNetMon is not able to calculate traffic for all your hosts in 1 second.
You may consider switching to more powerful CPU (single thread performance matters) or you can switch to advanced version which can do speed calculation much more efficiently.
Sincerely yours, Pavel Odintsov