Same here. More specifically, the log-in itself is working (user is authenticated and cookies/session variables are being set), but the return parameter is not being respected (the user is not taken to “m/s”).
{exp:member:login_form
form_class="form"
form_id="log-in"
return="m/s"
}
<ul>
<li>
<label for="mem-user">Username</label>
<input type="text" name="username" id="mem-user" value="">
</li>
<li>
<label for="mem-pass">Password</label>
<input type="password" name="password" id="mem-pass" value="">
</li>
<li>
<input type="submit" name="submit" value="Submit" class="submit">
</li>
</ul>
{/exp:member:login_form}
It was working until I upgraded to 2.6. o_O