Within your WHMCS admin area, navigate to Setup >> General Settings >> Other and check if you have Default to Client Area ticked.
http://docs.whmcs.com/Other_Tab#Default_to_Client_Area
Enabling Disable CC Processing would prevent invoices for a client, due via a merchant gateway, from being automatically attempted for capture; leaving them to login and make payment manually for all invoices, even with a credit card stored. The standard PayPal gateway is not a CC gateway; and, would be unaffected by it.
WHMCS already does send the IPN URL specific to your installation with the initial transaction. However, you must still ensure that IPN is configured/enabled within your PayPal account itself. It does not really matter much what URL you have defined at PayPal as the IPN URL since WHMCS does send it with the transaction; but, something must be entered into that field at PayPal nonetheless.
If you find false positives, you can disable individual rules for specific locations (i.e. files).
For example:
<LocationMatch "/some/path/to/file.php">
SecRuleRemoveById 390707
</LocationMatch>
That way, you're not deactivating the rule for the entire server or even an entire account; disabling it only for a specific file, while leaving it enabled everywhere else.
Assuming that the time zones are set correctly and that the times are properly synched on both devices/systems, timezones would be taken into account. If the time on either and/or the other is out-of-sync by too much, the tokens will fail.
If you are wanting to find the status of a client's products/services, using the Get Clients Products function, the status (i.e. Active, Suspended, etc.) of each of their products/services is returned within an XML response.
You may also be interested in the Licensing Addon for the licensing of your application.
Have a look at Maintenance: Resetting Your Database.
Note: Resetting your database only deletes client accounts and information. You will not lose any configuration or product settings you have setup.
Below are the available downloads:
* WHMCS V5.2.3 Stable Release [FULL VERSION]
*** To be used for all new installations and upgrades.
* WHMCS V5.2.3 Stable Release [iNCREMENTAL UPDATE]
*** Incremental version to be used for upgrading from WHMCS v5.2.1 or later only.
Be sure to download the correct version. If you are upgrading from v5.2.1 or later, use the incremental update. Otherwise, use the full version.
In your specific case, going from v5.1.5 to v5.2.3, you would want to use the "WHMCS V5.2.3 Stable Release [FULL VERSION]" download.
Upgrade instructions can be found @ http://docs.whmcs.com/Version_5.2_Release_Notes#Upgrade_Steps
The full change log for this release can be found @ http://docs.whmcs.com/Changelog:WHMCS_V5.2
Check the ticket mail import log (Utilities >> Logs >> Ticket Mail Import Log) and also verify your support department configurations. This can happen when WHMCS does not recognize the e-mail address as matching one of the configured support departments, defaulting to the first department.