calc is corrupting formulae when opened

I have opened a well used file that has not been a problem before, but now when opened it corrupts the formulae in the sheets. Additionally several sheets of information have been lost for example, if there are 6 sheets, the lost ones are alternate sheets e.g. 1, 3, 5. I’m using version 5.0.2. I have upgraded with the same results on the latest version. Any ideas please.

Do you repair crystal balls? Mine is broken.