I spotted this issue first on another forum, referencing this topic on the Avira forum: "Startup of the Application Blocked for your Security" for most applications. The next Avira update solved the problem for some users but not all. The problem was apparently with Avira ProActiv.
Avira KB article: How to add an exception for a blocked application.
Avira Antivirus update cripples millions of Windows PCs | ZDNet:
Avira KB article: How to add an exception for a blocked application.
Avira Antivirus update cripples millions of Windows PCs | ZDNet:
Avira has sent out a defective antivirus update that is causing paid versions of its product to block critical Windows processes and third-party software, effectively rendering millions of PCs unusable.