Versions of the Serial Port Arbitrator earlier than 2.1 can cause a freeze at startup when a serial port is shared with the DOS and PC Compatibility cards. This is stated in the Read Me document for the DOS Compatibility version 1.5 software.
The OpenTPT Serial Arbitrator, installed by Open Transport PPP, can cause the same symptoms. If you want to share the serial port with the DOS environment as COM1 or COM2, then remove both the Serial Port Arbitrator and the OpenTPT Serial Arbitrator from the Extensions folder. You could also disable these in the Extensions Manager. After the freeze occurs it is very likely you have a corrupted PC Setup Preferences file. Remove this from the Preferences folder and reconfigure PC Setup.
Serial Port Arbitrator version 2.1 should work fine if having it is necessary. This version is installed by both the Apple Remote Access client and server software version 2.1. The primary reason to have the Serial Port Arbitrator is if you have an Apple Remote Access server set to automatically answer, but want to be able to dial out without turning off the server software. The arbitrator allows the dialing application temporary control over the serial port.
More information about how the Serial Port Arbitrator works can be found in the following Tech Info Library article:
Article 11021: "
PowerBook: How the Serial Port Arbitrator Works"