update: i tried the install on a second machine. everything up and running.
difference between those 2 environments: the working one has rvt 2017-2021 installed
the other only rvt 2017-2020
any ideas?
hi i tried to install Dynamo core 2.8
the error i received:
this file or assembly “xxxxxx” or a dependency was not found. the procedure is not supported.(Exception of HRESULT:xxxx)
the version was downloaded and copied to a directory in parallel to 2.7, 2.6,…
what am i missing?

after that message i recived this one:
meaning DynamoSandbox could not be started, unhandled exception…
Do you have all Required components?
yes, i think so as 2.7 sandbox is running flawlessly
OK, next possible cause:
Did you use 7zip to unpack the zipfile??
no was using winrar. but did this on both machines the same way
My suggestions came from the GitHub Dynamo wiki page.
Maybe i skipped one option.
marcel, thank you for your reply. i followed the hints in the wiki.
finally i assume that some rvt 2021 library is necessary for the installation. as my two machines are identical but the rvt 2021 installed on one of them.
tonight i will install 2021 on the second machine and see if it affects the 2.8.0 sandbox
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please use 7zip as suggested by @Marcel_Rijsmus if you have not already. WinRar is not sufficient in many cases depending on the machines setup.
I agree with what @Michael_Kirschner2 has said though i would add that in some cases, right clicking the zip file then going to properties then setting the zip file to “Unblock” then unzipping with windows unzipper(not winrar) has worked fine for me.