I tested this locally and the following settings and template should do what you want it to do. A login form that shows the errors above the form, or redirects to a different url if there is an error.

https://www.dropbox.com/s/tb1p3o1girnvgi5/ticket-2122-csm.png?dl=0

This is the account.group/index.html file:

{if csm:error}
    {!-- Redirect ... but if you do this the error messages will be lost b/c it's a full http request. --}
    {redirect="pages/index"}
    
    {!-- Or show the error above the login form --}
    <div class="form-error-section">
        <div class="error-message">
            <strong>The following errors were encountered:</strong>
            <ul class="error-fields">
                {csm:content}
            </ul>
        </div>
    </div>
{/if}

{exp:member:login_form return="account/index" action="account/index"}
    <div>
        <label for="signin-email">Email Addresss <span class="input-desc">(Username)</span><span class="input-required"> *</span></label>
        <input type="text" id="signin-email" name="username" />
    </div>
    <div>
        <label for="signin-password">Password <span class="input-required"> *</span></label>
        <input type="password" id="signin-password" name="password" />
        <button type="submit" name="submit" value="Sign In">Sign In</button>
    </div>
{/exp:member:login_form}

Note that if you use an actual {redirect=”“} tag, or a php header(‘Location:’) function call to redirect, then the error messages will be lost b/c you’re doing a full http request. It only works when the “Redirect to defined error template” option is enabled.

I updated the docs page so hopefully it is a little more clear. I hadn’t noticed that over time my example screenshot of the settings didn’t align anymore with the example code snippets. Hopefully now it is more clear. https://docs.boldminded.com/custom-system-messages/docs/template-tags

Add this to line 358 of ext.system_messages.php

if (isset(ee()->TMPL->tagparams['csm_return'])) {
                $template = ee()->TMPL->tagparams['csm_return'];
            } else {
                $template = $template[1] == 'index' ? $template[0] : $template[0].'/'.$template[1];
            }

Should look like this: https://www.dropbox.com/s/1h133yaruk5f1e5/ticket-2122-new-code.png?dl=0

Then add the param to your tag and see if it works. This might let you set 2 different return urls for each of your forms.

{exp:member:login_form return="account/index" action="account/index" csm_return="account/index/foo"}

I tested my code change yesterday, it worked as I expected it to and allowed 2 different forms to redirect to different urls and display the error message above the form. Are you positive you tested it correctly?

Using RET is risky b/c it changes the behavior of that code, which has been like that for almost 10 years. I don’t want to break people’s sites by adding that functionality, which is why I added the csm_return parameter option. I probably won’t be making any changes that refer to the RET parameter. If that works for you though then you’re welcome to use it.

It’s hard to tell b/c I don’t know what else is going on with your site, but I step debugged it locally and csm_return was definitely in the tagparams array when it needed to be.