It is your fault!
Following your advice, I removed the e-mail log-in from both PayPal accounts, and added them to the PWM Username field.
At that point, I was able to log on to the new account, but not to my own account.
Took me a moment or two to figure out why: When I added my own e-mail address to the Username field in PWM, that changed my password.
How to fix that: Return to PayPal, and, in the proper field, tell PWM to populate that field; then, in PWM, remove my username, which reset my password to its original, then add that password to the PWM Password Prefix box, thereby overriding PWM's attempt to reset my password when I reinstalled my e-mail in the Username box and removed the auto-populate entry.
See, I told you it was your fault....
(We will just ignore the fact that quite a few of my older PWM entries were done improperly, by my not having entered my e-mail address in the Username box; this was true for Paypal, so when I looked at that PWM entry and created the new one earlier today, I did it the same way as I had quite some time ago for my own PayPal account. Of course, you also realize that, had you told me about doing this correctly quite some time ago, the problem with PayPal would not have arisen today, don't you?...)
All of my somewhat-incorrect PWM accounts work fine even when I failed to enter my username; PayPal is the first account for which I've needed to create two separate accounts, so the "Choose Account" problem never arose before today.
Therefore, even though we must acknowledge that whatever it is that is your fault is your fault, nonetheless, you have redeemed yourself by pointing out that I was at fault by putting the e-mail address in the wrong place.
Problem appears to be solved. I have, as is my usual procedure, made myself a note to send you a thank-you. If you don't receive that, well, it will be your fault for not reminding me to send it.
Commodum ex iniuria sua nemo habere debet - No person ought to have advantage from his own wrong
[Addendum: As I was previewing this, there was a notification that a new reply had been posted. As what I wrote above, yes, I was doing things incorrectly for many of my accounts. As for PayPal, I already have two separate accounts, and by following your advice, as I stated above, the problem has been solved. So, again, my thanks for the advice and assistance.]