There's no problem to save and open in same version.
If I save in same version I can not open again and work in the pages. I must take file/export/ open in IDML
Is something wrong with my program? or shall I export and open in IDML?
Most versions with dots in the number are updates that are considered the same version. What I mean is that version 23 is the same as 23.1 and 23.63. The dots indicate an update to fix bugs in earlier releases of the number to the left of the dot. New versions get a larger number to the left of the dot, so version 24 would be newer than version 23, and with most programs, version 23 wouldn't be able to open files saved in version 24.
That said, InDesign 5 and 5.5 are not the same version. You shouldn't expect to be able to open 5.5 files in 5. What you can do is export from 5.5 as IDML and open that file in 5. It sounds like you understand that, but the way you wrote: "open in IDML" is a little confusing. You don't really open in IDML, the IDML is a file format that you open the same way you open a regular InDesign file. I think you already know what to do, so I'll just say that it is working the way it's supposed to.
By the way, the best workflow is to stay in the same version the whole time. You can make a document in one version, and at a later time with a later version, you can still use the document, but sending it back to the old version is a bad idea. If you need it to be in the old version, just don't open it in the newer version. The reason is that it may not look the same as it did after you move it back to the old version, you might then have to do some tweaking to get it back the way you want (and hope you didn't miss anything).
It's not clear to me if you are trying to reopen in the same version of ID on the same computer, or if you are moving the files to another computer...
What error message do you receive when you try to reopen the file?
Suzanne, your last post was blank. That happens if you try to respond by email with some email programs.