when I try to open a file in calc, I get a "text import" window

Win 7 Pro OS. LO ver 5 en
How do I get out of the “text import” window & back to my file?

2017-11-21 new development on Windows XP, LibreOffice 5.0.3. I upgraded to LO 5.3.7 as a first attempt at trouble shooting, to no effect.

If I am the first to open the file, it opens normally. if someone has it open already, LO presents the “text import” dialogue. Regardless of response, the file never opens. If I choose OK, it (logically enough) just fails. If I choose cancel the “document in use” dialogue shows up, but “open read-only” and “open copy” both fail (scalc.exe terminates).