In addition to qubits post:
You can look for a lock-file in the same directory as the xls-files. Lock-files are created by LO upon opening a document. When the user gets disconnected from the network or his LO isn’t closed properly, these lock-files persist and give a false error message when trying to open the file.
The lock-files are hidden so, you may need to enable the view of hidden files in your file-explorer. The naming of the lock files is as follows:
.~lock."Name-of-XLS".xls#
whereby “Name-of-XLS” is the actual name of XLS-file (w/o the quotation marks). - When you find such files, just delete them and try opening the files again.
Another solution would be:
Open the files in LO (read-only), choose File - Save as … and save them in the OpenDocumentFormat (ODF), which would be here: ODS for spreadsheets, ODT for textdocuments. When working with LO this is the recommended file-format (anyways).
HTH