When I save a spreadsheet in XLSX format and try to open it on my WindowsPhone 8.1 office app, I get a message “unable to open - unknown format”.
LibO Vers. 4.1.6.2
OS Windows 7 Prof
Please include a question 
I think the only way to get a quick solution, try a more recent version of LibreOffice. A lot of work was done to achieve a better compatibility.
Now I am using LibO 4.2.5.2, but still facing the same problem. Should I report a bug?
Please do it. Remember attach a sample file with the issue.
Try to save the file as .ODS and load it in the OfficeApp. MSOffice can open this files but it might complain that it is corrupted.
Try also the older version .XLS if possible.
Can the OfficeApp open XLSX formats?
MSOffice on WindowsPhone cannot open .ods files
.xls files (MSOffice 97/2000) can be opened.
When I convert .xls to .xlsx using a converter program, the .xlsx can be opened.