Created attachment 168419 [details]
sheet without conditional formatting

(In reply to VLB from comment #0)

> When I have a sheet with multiple conditional formatting and save it in
> excel 2013 as *.xlsx and then open in excel 2013, it gives an error.
> When removing all conditional formatting and saving again as *.xlsx, the
> file will still open without an error message.

The sheet is saved in LO and when opening in excel 2013 give the error.

Step to reproduce;

1) open file "sheet with conditional formatting" in LO
2) save as XSLX and open in excel and there are error present

Reproduced in

Version: 7.2.0.0.alpha0+
Build ID: 6156a419a2d3f09d91afd00f84c84ba717442f43
CPU threads: 4; OS: Linux 5.7; UI render: default; VCL: gtk3
Locale: en-US (en_US.UTF-8); UI: en-US
Calc: threaded

Also reproduced in

Version: 5.2.0.0.alpha0+
Build ID: 3ca42d8d51174010d5e8a32b96e9b4c0b3730a53
Threads 4; Ver: 5.7; Render: default; 

Locale: en-US (en_US.UTF-8)

and

Version: 4.3.0.0.alpha1+
Build ID: c15927f20d4727c3b8de68497b6949e72f9e6e9e

Created attachment 168460 [details]
sheet with redused conditional formatting

I checked the conditions and there were 2 conditions that referred to a text.
This formula did not have to be stated med/mv;rd>1 , but "med/mv;rd>1". This is an incorrect condition that must be stated with quotation marks, of which no message is given and the error message is only mentioned when opened in Excel.

The problem also occurs with wrong ranges, ranges and names.

Created attachment 168736 [details]
Example file created from scratch to demonstrate the problem

(In reply to NISZ LibreOffice Team from comment #9)
> Created attachment 168736 [details]
> Example file created from scratch to demonstrate the problem

Oops, too soon.

This file shows that a conditional formatting First/Second;Third>1 in B3 is valid conditional format in ODS without quotes, if First/Second/Third are valid named ranges (pointing to A1/A2/A3).

Also fff/sss;ttt>1 in B5 is valid in ODS without quotes when fff/sss/ttt are not valid named ranges.

Saving this file to XLSX will generate invalid contents detected error in Excel upon opening.

Putting these conditions into quotes as shown in B4 and B6 does not generate invalid contents in XLSX.

B1 and B2 shows that the two parts of the condition "First/Second;Third>1" are valid without quotes in XLSX too, only connecting them with the semicolon makes it necessary to quote them.

D3 shows that using direct cell references instead of named ranges like $Munkalap1.$A$1/$Munkalap1.$A$2;$Munkalap1.$A$3>1 
is also valid without quotes when saved in XLSX.

In my original sheet the problem is no longer present, but in the added test file the problem is still present.