Well, just in case anyone else is stuck with the same problem and comes across this old string: After several tests I have come up with this solution that works for me - and perhaps for others too:
Notice that the subtitle borders are set differently than the default settings. The settings I use keeps the subtitle string just inside the outer safe area border where as the default settings are inside the inner safe area border and thus resulting in shorter subtitle lines.
You can choose a different typeface and size if you like. These matters doesn't seem to have much influence on the "_too complex_" issue.
With these settings I can writea maximum of60characters, including spaces, per line. Remember to makeforcedline breaksbetween the lines,ifyoursubtitle stringislonger than one line. Otherwise Encore will see what appears to be two lines as one long line exceeding the max number of characters and thus resulting in a "_too complex_" error.
And as for number of frames between subtitle strings: Minimum 5 frames - but Encore will automaticly warn you if you place text strings closer than that.