Thank you for posting the tournament - it may prove helpful. In my latest dev version, there's no error when displaying the Tables tab. The player at "Table 18" in seat 2 is "unknown". On the Seating Chart screen, you should see it as <Unknown Player>. As you can see, I've accounted for cases where there is a player the TD doesn't have any knowledge of, but for the life of my I cannot remember why, or imagine a scenario where this might be reasonable. Attempting to drag this player from the seat causes another error, and I'll fix that.
The key will be how you got into this situation and correcting it at the source. So the internal ID of the player in that seat is "aafdb590-5732-11e0-17fe-0d5a0ebdc4a1". I would tell you to go to the Database tab and do a search for "aafdb590-5732-11e0-17fe-0d5a0ebdc4a1", but I don't see why you would find anything. If the player was in the database, you shouldn't be having this problem.
Try this instead: Press Ctrl+Alt+Shift+V to open the "System Information" dialog. Look for the "DB Folder" line, right-click it and select "Explore folder". Sort the folder by name and look for the file "aafdb590-5732-11e0-17fe-0d5a0ebdc4a1". If you find a file you can double-click it and open it with Notepad. This will contain the player's detail information (address, phone, email, etc), but not their name. Maybe that can help you determine who the player is.
Either way, have you any idea what transpired before this that might have landed you in this situation? From what I can tell, you've got a seated player that isn't in the tournament or your database. Any idea how it happened?