Hi
if you edited a clip using the red Start and blue End markers in Clip Preview, the clip on the timeline is truncated in real time and the timeline adjusted to remove any gaps.
If, on he other hand you selected the Sequence from the Sequence Clip bin (which in effect is the whole of the timeline) and used the Start and End markers, the timeline is not readjusted (like a clip) but the truncated section is rendered grey and will not export.
To correct this, click into Clip Preview mode, open the Sequence clip bin and left click the sequence. It will open in the preview pane and you should now see that the red Start marker is not at the start of the sequence and the bit to the left of it will be greyed out. Grab the marker and move it left to remove the grey area.
The timeline should now look normal and export correctly.
Nat.