Are there many such troublesome files?
You could try using
http://www.zamzar.comfree online service to convert these to .odt format.
It may be that these particular files have become corrupt, particularly if transferred from an older computer. Until you have retrieved the data within them, under no circumstances should you Save over them - use a different name.
Apache OpenOffice 4.1.9 on Xubuntu 20.04.1 (mostly 64 bit version) and very infrequently on Win2K/XP