This is an improved and complete fix of #4971. Copying the testing instructions to here:
Steps to reproduce the issue
Install Joomla (Test English (GB) Sample Data), for example
You will have 3 times the error:
The proposal to fix is to replace
Notice: Array to string conversion in /libraries/cms/router/site.php on line 465
System information (as much as possible)
I further cleaned up the router. Basically it should work the same as before, except that that notice is gone. And we have quite a bit less code.