Welcome to the Community. I can help you connect your client's PayPal account to their company file.
I checked to see if there are any reported cases similar to yours, but I found none. Let's see if we can link their PayPal account in a private window. These will help us determine if the data on your cache is causing this hitch.
Here are some keyboard shortcuts you can use:
In Mozilla Firefox and Internet Explorer, press Ctrl + Shift + P.
In Google Chrome, press Ctrl + Shift + N.
In Safari, press Command + Shift + N.
On the Incognito page, you can follow these steps to connect the Sync with PayPal app to your QuickBooks Online account:
Select Find Apps, then enter Paypal into the search bar.
From the choices, choose Connect to PayPal.
Tap the Get App Now button.
Answer the interview, then select Next.
Hit Connect to Paypal, then click Let's do it.
Follow the remaining steps to connect and set up the app settings to your choice.
If you can connect your PayPal account, open the non-private browser again, and delete the temporary internet files stored in the cache. If the same thing happens, please use a different browser, so we’ll know if this only happens to one browser.
That should be it. I'm confident this resource will help you with connecting the account successfully. Please update me if you have any questions on how to use the PayPal app. I'm here to help if you need further assistance. Have a great day!