0x1B1 - Version mismatch between executable and preexisting shared memory versions! EXITING. You may need to stop RTVSCAN or reboot.
It is documented as SPR# DDES7L9SEV, and IBM thought they had it fixed in 8.0.2 FP2 and 8.5 FP1. SPR #TONN7WTQQE was opened based on reports this is happening in 8.5.1, which I can confirm it really is.
Adam called Lotus support and they had him uninstall Notes, manually remove all the directories and Registry entries, and try again. It still failed with the same error. He sent in NSD's and other logs and there was no resolution. The final response he received was that it is under investigation. At this point he is stuck not being able to install 8.5.1 at all on his computer, which means he can't test his applications for compatibility.
This morning the same thing happened to me. I upgraded a test VM from 8.5 FP1 to 8.5.1 and got the same 0x1B1 error as Adam. I tried the same cleanup he did but it didn't help me, either. I'm writing this while I'm waiting for Windows to install in a new VM so I can see if it was just a fluke. Whether it is or not, this hardly inspires confidence.