Description of Problem:

After using the "Select Thread" function the Go To 
Next/Previous unread messages doesn't work

Steps to reproduce the problem:
1. select a message in a thread
2. select Thread (Edit > Select Thread or Ctrl + 
H)
3. Go To next/previous Unread message 

Actual Results:
nothing happens, the thread remains selected.

Expected Results:
The next unread message should be selected and/or 
previewed

How often does this happen? 
every time

Additional Information (Thanks Guenther):

This actually is cause by Select Thread. Selecting 
multiple messages manually (even if it is the 
entire thread) and going to the next unread
message does work fine.

Workaround (Thanks Guenther):

Selecting another mail (after step 2.) by arrow 
up/down, makes the accelerator to go to the Next 
Unread Message work again.