SQLite3 Database suddenly reported as corrupt or invalid file format

I have a SQLite database on a Ubuntu 14.04 machine that I suddenly cannot read with either the Firefox extension SQLite Manager or SqliteBrowser from my Windows 10 machine. I was able to do it yesterday and cannot do it today. These are the error messages I get:

• "SQLiteManager: Error in opening file company_data.sqlite - either the file is encrypted or corrupt Exception Name: NS_ERROR_STORAGE_IOERR"
• SqliteBrowser just says "Invalid file format"

I ran pragma integrity_check; and it says the database is OK. I can access the database normally from the command line on the Linux machine and also with my python scripts.

My sqlite version on Ubuntu is 3.8.2 and on SqliteBrowser is 3.9. What could cause this problem and how can I fix it?