503 when trying to get to admin pages

Does the error occur if you manually log in as a subscriber and perform the same action?

If your server is throwing a 503 then this is likely due to some sort of server misconfiguration, possibly due to cookie handling or any one of an infinite number of other possibilities.

Using a dummy user directly works fine. I am sure it is not the server but some internal loop where Apache eventually detects this and shuts down the request.

This is going to be pretty hard for me to debug. This is the only report of this problem in 6 years and 250,000 downloads, so I doubt the plugin is directly responsible. It could well be that one of User Switching’s cookies is triggering something in your WAF or Suhosin.

Any errors showing up in your server error log or PHP’s error log?

I just had this same issue. My logs had this error:
upstream sent too big header while reading response header from upstream

Increasing the fastcgi buffers fixed the issue. More info about that here.

@mindctrl: Interesting. User Switching’s cookies are typically the same size as WordPress’ authentication cookies, and they’re pretty small. You might want to check to see what other headers are being sent by your server, especially if the headers don’t fit into 4KB.