Add new Subtitle quits working

Encore CS3

Two Subtitle tracks, each created by importing a text file.

I have added several additional subtitles in the standard manner, and been able
to edit and save imported text with no problems.

Suddenly, I cannot add a new subtitle to an active track. I can still edit
existing subtitles, but when I position the playback cursor over an empty space,
and try to add a new subtitle, the mouse cursor will not change to an I-bar, but
stays as an arrow.

I have saved, rebooted, opened one, then each subtitle track all with no
change.

I have about 2800 subtitles between the two tracks. Is there a physical limit,
or should I bee looking elsewhere?

Since I only have one or two more subtitles to add, getting this fixed would be
a help. If you've worked through this issue, please let me know.

Never mind.