Multiple Problems with Mozilla Products: Thunderbird and VPN

Within the past 4-5 days, I’ve been unable to use Thunderbird because I get an error message that I cannot get past that prevents all other TB actions. The message is:
“The messages could not be filtered to folder ‘Deleted on’ because adding a message to it failed. Verify that the folder is displaying properly or try to repair it from the folder properties.”

First, I don’t have a clue how to proceed because the message is non-specific because it gives no clue how to repair from folder properties. It seems more inane given that Thunderbird is locked by the error blocking any further action, which I presume would be done in TB to repair folder properties.

Second, I can’t access mail to any other accounts because the one account problem prevents it.

Third, I can’t use online resources to address the problem because most require an email exchange