Probably because the login posts to PostLogin which then gets redirected and in that redirect your posted values get lost.

First of all: I'll make an assumption that you're doing this for SEO purposes.. in which case you should probably stop listening to SEO swindlers, the casing of the URLs really doesn't matter ;-)

But if you must do this you can to fix this:

<rules>
    <rule name="LowerCase" stopProcessing="true">
        <match url="[A-Z]" ignoreCase="false" />
         <conditions>
          <add input="{URL}" negate="true" pattern="^~/umbraco$" />
         </conditions>
        <action type="Redirect" url="{ToLower:{URL}}" />
    </rule>
</rules>

You might still run into other unexpected problems though, good luck! :-)

Hmmm it doesn't match the urls for some reason and on the basis of I personally hate Regex I think I'll just leave it out...

I had this problem today and found a solution:

<rewrite>
    <rules>
      <!-- Redirect rule to force all URLs to lowercase -->
      <rule name="LowerCaseRule" stopProcessing="true">
        <match url="[A-Z]" ignoreCase="false" />
        <action type="Redirect" url="{ToLower:{URL}}" />
        <conditions>
          <add input="{REQUEST_URI}" pattern="^.*/umbraco" negate="true" />
          <add input="{REQUEST_URI}" pattern="^.*/install" negate="true" />
        </conditions>
      </rule>
    </rules>
  </rewrite>

Do you have a version of this rule that works with ClientDependency? I'm having issues with tolower, I just can't seem to get it to ignore anything from ClientDependency.

I've tried adding

<add input="{REQUEST_FILENAME}" pattern="DependencyHandler.axd" negate="true" />
<add input="{REQUEST_URI}" pattern="(.*?)\.axd$" negate="true" />

etc..

but it doesn't work. When I try and login to umbraco back end, I can see in console that the urls have been modified by towlower and PostLogin method is showing error 405

I also have the above listed /umbraco / install condidtions.

(on umbraco version 8.6.1)

I have figure out the answer..

Apply the folder exclusion to umbracoapi folder and it all plays nice..

<add input="{URL}" pattern="umbracoapi" negate="true" />

So far! :)

is working on a reply...