Error 500 after upgrade

[Jugoslav]

Hello,

I have a AsyncUploadHandler that works fine in an older version 2019.1.215 but as soon as I upgrade to newer version e.g. 2021.1.119 I start getting error 500

I doubt that my handler is still referencing the old version but can't find where it happens and change it eventually. 

Any ideas?

Thank you very much