When I delete a number of messages ...

How many messages are "a number of messages"?

... then empty the trash, ...

How many messages are in Trash when you do that? Does emptying the trash actually work?

... I cannot delete another message until I restart Thunderbird.

What happens when you attempt to delete another message?

I delete a message, empty trash there are 818 messages in trash, trash folder is empty. I attempt to delete another message and get the following error message: "There is not enough disk space to download new messages. Try deleting old mail, emptying the trash folder, and compacting your mail folders, and then try again." I can download new messages. Emptying the trash folder and attempting to delete another message is what prompted the error message. When I attempt to compact my mail folders, I get the following error message: The folder "inbox on *****.net" cannot be completed because another operation is in progress. Please try again later." I can clear the error messages and resume normal operation only after closing and restarting Thunderbird.

What is your anti-virus software?

Try to start Windows 7 in safe mode with networking enabled.

Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

Antivirus is Malwarebytes premium. Restarted in safe mode. Emptied trash containing 980 messages. I get the same error message.

I just deleted 5 messages and emptied trash. When I attempt to delete another message, I get the same error message. When I attempt to move that message to trash for this folder, I get the same error message. When I move that message to the trash subfolder for another folder, the message is moved to that trash folder,

I just deleted 502 messages from the trash subfolder I was moving messages to after being unable to trash them into the trash subfolder from the folder I am working from. I can still move messages to this second trash subfolder. Is there a limit to the number of messages that can be emptied from the trash folder?

There is no limit. You shouldn't be having this problem.