Can you please record a screencast in which you:

• Start WinSCP
  
• Go to Window page in Preferences (and close it)
  
• Back on the main window, close all the tabs, except for one
  
• Close WinSCP
  
• Start WinSCP again
  

Thanks.

Actually, it does not overwrite. Doesn't not matter how few or many tabs I close before exiting. It always begins anew with the errant 100+ tabs open message.

I'm sorry for the complications.
The workspace does not save/overwrite, when there's nothing opened. Keep at least one tab opened when you are closing WinSCP, so that the 100-tab workspace is overwritten.
There's indeed no command to close all tabs at once, no one asked for that yet.
It would be quicker to close the tabs using Ctrl+Shift+D (or Ctrl+W) keyboard shortcuts than using mouse.

So, I follow your instructions to Show Tabs. They appear, so I X all 100+ them, tediously one at a time (Can I close them all with one click, and never see them again? They seem to do nothing for me except to cause trouble.) After closing the last tab, the Login dialog appears (see attached Capture5.PNG). I close it, then X the application. I re-launch the application. I get the same errant message again (Capture6.PNG).

I OK the errant message. Viola! The application's main window appears, with the 100+ tabs that I thought I had already closed (Capture7.PNG).

Question: How do I stop the application from opening these crazy tabs?

Ok, so in your original post by the "two tabs" you probably meant the two file panels.

I believe you actually have over 100 tabs/sessions opened. But as you have the tabs hidden, you do not see it. Go to Options > Tabs to (re-)show them.
https://winscp.net/eng/docs/ui_tabs

It seems that each time you use WinSCP, you open a new session/tab, instead of re-connection the existing one.

Attached images as requested.

Thanks for your report.
Can you please post screenshots of the main window (with the two tabs) and of the "unending submenu of identical tabs"?

Every time I open WinSCP 6.3.1, I get the prompt:

Do you really want to open another tab? You have 100 tabs opened already. Please consider closing some tabs first to free resources of your computer.

Don't know what it means. Appears even after rebooting. Didn't have this issue with previous version. Everything apparently works OK after I respond by clicking OK.

After getting in, click on main menu: Tabs > Opened tabs, I get an unending submenu of identical tabs, although only have two sub-windows (tabs?) open, one for my local computer and the other for the remote server computer.

Don't recall opening any tabs. Don't know why I need "tabs." See no way of closing the apparently 100+ tabs.

Commander interface. Never happened before with earlier versions.