Depending on the length of the replay, the initial dirt screen may last longer than usual.
If it's still there after several minutes, install this mod (drop it into your mods folder). Then, whenever you start Minecraft, another small window will open which has a button labled Create Thread Dump
. Some time after the dirt screen appears, press that button. It'll generate a thread dump in the text area above the button. Copy that thread dump and pastebin it. It's probably best if you create multiple (~5) of those thread dumps each with a short time (a few seconds or however long you take to pastebin it) in between.
Yes, that may very well be the same issue.
This seems to be caused by a world change during or shortly before the time in the replay which you're rendering. is there such a world change?
If so, the workaround would be to render two separate paths, one in each world (or if there's a world change right before the rendered time, start the path a few seconds later).
Ticket for reference: https://bugs.replaymod.com/show_bug.cgi?id=89