Customer visits website and they are able to login, but when they logout, they are unable to login again. When they click login, the login page just refreshes.
Additionally, if i clear the site cache using our Supercacher utility located in our Siteground control panel, the customer is then able to login again, but if they logout, they are then unable to login again.
This might be happening due to cookie or formkey issue.
1. Form Key missing
Magento 1.9 is using form_key validation at customer log-in and registration pages.
when we click on login(post action) button it check this form_key using _validateFormKey()
To fix this issue add this code to your login form(wherever it is) /app/design/frontend/yourtheme/template/persistent/customer/form/login.phtml
<input name="form_key" type="hidden" value="<?php echo Mage::getSingleton('core/session')->getFormKey() ?>" />
you should add it right after <ul class="form-list">
2. Cookie issue
Go to Sytem -> Configuration -> Web -> Session and Cookie Management
Try increasing the cookie life time to 86400
