When I am in Vegas 17 and right click a clip to and select Edit in Vegas effects I get " An error occurred during the current operation. The file could not be created." Note I am using the correct serial number for Vegas Post.
I have the the same issue, or rather 2 issues:
1) if I install vegas post according procedure, and start vegas 17 in normal (non-administrator), then I have vegas effect running in demo version
2) after starting vegas 17 in admisnistrator, vegas post tries to launch and crashes, or I just get your warning.
I submitted a ticket to Magix.
What you could do is go into windows >> Control Panel\System and Security\Administrative Tools
and launch the event viewer, goto windows logs select application, then filter on event-id 1001 (windows error reporting) and report back (if it is an vegas effect APP crash)
There I have seen in my case that maybe an older Boris OFX is clashing with vegas effects.
I still need to find out which older Boris OFX to uninstall...
https://www.vegascreativesoftware.info/us/forum/vegas-post-vegas-effects-only-demo-version--116739/