I am getting below html page,while accessing the oauth manager page:
Below are the logs :
I remember it's a known bug.
You actually have error on OTK, the OTK returns an error page, its content-type should be html, but uses wrong content-type as plain text, therefore the browser shows the html source code.
You can try to save to html code as a xx.html file, and open with browser, then you should see the error the OTK returned.
I tried to install OTK4.0,4.1,4.2, still getting the same issue.
Okay, this is the correct error page it should show (not html source code), but the error page doesn't tell much about the error.
First of all, you may double check if the post-install tasks have been done,
Post-Installation Tasks - CA API Management OAuth Toolkit - 4.2 - CA Technologies Documentation