I have trouble with the formatting when viewing an invoice or a quote
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You probably have the bridge helper addon for WHMCS installed and pages are redirecting to your Wordpress site. For viewinvoice and viewquote, WHMCS uses a non formatted page and this causes the redirect to produce some strange results or no results at all.
To rememedy this, you can disable redirect for viewinvoice and viewquote. Just tick the relevant option in the bridge helper addon settings in WHMCS.