OK, I just downloaded and installed version 5.4.9.18 from the link below: https://ipcamtalk.com/downloads/blue-iris-updates-64-bit.3/version/871/download I then let it automatically upgrade to v5.5.3.4, imported my registry and my C:/Blueiris folder back, etc. and all seems to be working perfectly n...

I have exactly the same problem. Did all the usual things like sfc /scannow, deleted HKLM and HKCU Blue Iris registry, checked drivers and OS updates, etc. all fine. Uninstalled Blue Iris completely (with registry) and reinstalled - still quits with ucrtbase.dll error straight after "Creating W...