The problem turned out to be a missing file, com.apple.ByteRangeLocking.plist, which is supposed to reside in the top-level /Library » Preferences folder. On my Mac Pro, it was missing (as was /var/db/BRLM.db, another file sharing-related file). The fix? You can copy the file from another Mac. However, it's a simple file, and forum poster voldenuit posted the contents:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>DB_HASH_TABLE_MAX</key>
<integer>4096</integer>
<key>DB_INIT_SIZE</key>
<integer>2097152</integer>
<key>DB_Location</key>
<string>/var/db/BRLM.db</string>
<key>DB_PROC_TABLE_MAX</key>
<integer>1000</integer>
</dict>
</plist>I copied the above code, ran sudo vi com.apple.ByteRangeLocking.plist (from the /Library » Preferences directory) to open the editor as root, then I pasted the code and saved the changes.
When I restarted Personal File Sharing, it worked perfectly: the BRLM.db file was created automatically, and I could once again login as myself without crashing the server. Hooray!

