This happens when Ping Plotter is trying to create a timer - and the Windows API call to create that timer fails. Timers are used for a few things - including doing the next sample (based on the trace interval), auto-saving (images and data), alerts, and a few other misc. functions.
This problem occurs on a PC running Windows98 with PingPlotter v2.20. This PC is connected to an ISP via a ISDN line. The person experiencing this problem felt that there might be a correlation between network failures and repeated occurrences of the error message 「Not enough timers available」.