Gmail and Claws

[shlomo solomon]

Google/Gmail has decided to drive me crazy and I hope someone can help.

Some background:
1 - my gmail address is shlomo.solomon at gmail.com
2 - all my mail is forwarded to my domain shlomo at the-solomons.net
3 - For MANY years, I've used Claws mail (and before that KMail) as my
client because I HATE webmail in general and Gmail specifically.
4 - Claws accesses my mail over POP3 and SMTP
5 - to allow this, I have Gmail set up to allow POP access and my
Google account set up to allow "Less secure app access" (Google-speak
for anything not provided or controlled by Google).
6 - This setup has worked for at least 20 years and I've made no
recent changes so there's no OBVIOUS reason for it not to work now.

BUT, in the past few weeks, Gmail has randomly refused to let Claws
access my mail. Sometimes this lasts for a short time and sometimes
for hours or even a day or more.

The Claws log shows:

* Account 'GMail': Connecting to POP3 server: pop.gmail.com:995...
[21:49:25] POP< +OK Gpop ready for requests from 89.237.110.180
s20mb165349719wra
[21:49:25] POP> USER shlomo.solomon at gmail.com
[21:49:25] POP< +OK send PASS
[21:49:25] POP> PASS ********
[21:49:25] POP< -ERR [AUTH] Web login required:
https://support.google.com/mail/answer/78754
*** error occurred on authentication
*** Authentication failed.

I've tried all the solutions mentioned on
https://support.google.com/mail/answer/78754 and many more that I've
found in various posts and sites, but nothing seems to work.

Among other things, I've tried changing my password, turning POP
access off and then back on, turning "less secure" off then back on,
and more.

The only thing I HAVE NOT tried (because I'm afraid it will make
things worse rather than better) is to  set up two-factor
authentication and use an app password - I also have no idea how this
works (or doesn't work) in Claws mail.

And as I wrote above, after a while, the problem solves itself.

And one more thing - I have additional Gmail accounts with the same
setup and Gmail DOES allow Claws mail access, while denying access to
my main account. So that's also weird.

Any help would be greatly appreciated.