Here are a few more suggestions to help troubleshoot your files.
1. Try the process with only one store and its customers. If it errs out, continue to step 2. If it works, this may be an issue of system resources.
2. Continue working with just one store and its customers until the process works.
3. Export out the store/cust layers as shapefiles (or gdb) and add them back to the toc. This does two things: It makes sure that the column headers are acceptable, i.e.:</=10 characters long, no spaces before, after or between any words, no punctuation save underscores, and do not begin with numbers. And, if missing, exporting the layers and readding them to the toc will create an OID field. Retest. If it errs again, go to step 4.
4. Make sure you are working with formal customer and store layers. If the layers have not gone through the store/cust wizards, bring them through. Retest. If it errs again, go to step 5.
5. Let BA create a store assignment as a test. This is done after both layers are created, via Customer Set up > by closest store assignment > tag the store layer > finish. Retest. If it errs again, go to step 6.
6. Convert the store/cust layers to gdb and retry. If it errs, go to step 7.
7. If you are choosing to create the distance field using either drive time or drive distance, try again using straight line. If the driving calcs err but the straight line analysis work, verify that you have Network Analyst installed and registered.
8. If you are using driving calcs and NA is registered, try breaking up your files. It may be that you just need to run fewer stores and their customers at one time.
9. If you are using driving calcs, make sure your temp drives are cleaned out. (Right click on Start > Explore > right click on Local Disc C > Properties > Disk Cleanup).
Please let me know how things go for you.
Best Regards.
Kristen C.
ESRI Support