Unfortunately, I'm getting the same problem in 1.5.4, on an early 2011 Mac Book Pro with 2.2 GHz Intel Core i7, 8GB memory, running OS 10.7.1. I'm processing a batch of 2092 images, with 35 keyframes across sunset exposure transitions.

I noticed the problem after an "Auto transition" skipped calculations on a few transitions. When I went back to apply linear transitions to these sequences one-by-one, I got a spinning pinwheel, forcing a hard quit. This happened multiple times on multiple transitions, even after a reboot and a fresh start with no other major apps running. I finally resorted to entering keyframe exposure data in Excel, then manually calculating and applying the exposure data for these images.

Okay, let's try to sort that out.
There are some things you could try out so that we can maybe narrow down the problem.

First: When using auto transition the keyframes should not be too close otherwise there might be gaps in the transitions. Thats known and not really a problem. Just leave some more frames between the keyframes. Exception are adjacent Keyframes to make hard changes on exposure changes (holy grail).

Regarding the force close: please check if there is anything in the system log.

Since it seems to occur on large image sequences (several thousend) it could as well be a memory issue. In the LRTimelapse folder there is a file plist.info, you can edit it with the associated tool on mac. It gives you the ability to change some parameters for the virtual machine including the memory reserved by the java virtual machine.

You are going to find the parameters:
‹string>-Xms256m –Xmx2048m</string›

the first value is the initial heap-size, you can increase it as well, but normally it's not necessary. The second is the maximum heap size.

Now you can replace the 1024m with a value (in MB) of memory you want to provide to LR-Timelapse. For example if you have 6GB ram, you can try
-Xmx2048m or –Xmx4096m

Please take care to not set it higher than the amount of RAM you can provide.

I hope this will help you. Please let me know if it did.

Best regards
Gunther