Wrong digital signature happens when the file CRC check in SBO-Common (SARI.InstChkSum value) does not match the .exe file on the machine. This means that in the system something is tampering with our installer.exe.
These are some steps that you can troubleshoot the problem with, also run the Business one client as an administrator, with UAC disabled and all the antivirus and firewall software services in disabled mode.
1) Disable any antivirus/malware/firewall program on the machine while installing
2) Check the machine for viruses (if some virus modify the installer .exe then it could give this error)
3) Verify SAP is installed on C: drive (there have been some issues when SAP Client was not installed on default drive)
4) Make sure the user has access to writing to the %temp% location
5) Check that if you go to "C:\Program Files (x86)\SAP\SAP Business One" and highlight the file "AddOnsLocalRegistration.sbo" that Windows does not have the "Compatibility files" menu item in the "Organize, Open, New Folder toolbar"
6) Check that SAP Windows Service "B1 Client Agent" is running
7) Check if on yet another machine you can install B1UP (will check if it is the data in SBO-COMMON that is wrong)
8) Reinstall the Add-on
9) Check this note from SAP https://launchpad.support.sap.com/#/notes/2990705