MS Office 2019 (on local machine, not Office 365)

I've been getting a lot of spam recently, so I created a few rules directly targeting the offenders. But now Outlook is blocking nearly everything and I haven't been able to figure out why.

I had a rule in there for my niece, because none of her recent messages were getting to me. I just had it send all messages from her to my Inbox (which it should have been doing anyway, because I have never had a rule blocking her messages). It didn't work. This is what Outlook 2019 has for the rule when I look in Manage Rules and Alerts:

Apply this rule after the message arrives
from [my niece's address]
move it to the Inbox folder

Nothing else.

When I exit Manage Rules and Alerts I get a message stating that "The rule has a condition that the server cannot process. The action 'stop processing more rules' will prevent all remaining server rules from being carried out." But it doesn't say which rule is doing this or where it is.

I am tempted to delete all my rules, but some of them go back decades, through many old versions of Outlook. I am afraid that, as soon as I do, I will lose all the rules I have carefully set up over the years and be innundated with spam (and possibly grant access to those that may have contained a virus -- though I do have one rule in there to block messages labeled "VIRUS", which Avast generates when it finds one.)

I can check my mail server and I see that all the missing messages are arriving there. Outlook just isn't downloading them. But using webmail is a pain, because I have to check each of my email accounts separately and I have to respond in webmail, rather than using Outlook and quoting the original message. What a pain! Any ideas on the best way to address this (the safest way -- and, if possible, the easiest to set up.) Absent a better idea, I'm going to have to delete all rules and then catch the bad ones and blacklist them one-by-one, not knowing whether I will unwittingly create the same problem again by doing whatever is causing this.

Thanks in advance. The last time I had a problem with Outlook, this forum was where I finally got the answers that eluded everyone else.