Calc does not ask for password to open a password protected xlsx file

Sometimes I get password-protected .xlsx files, which previously, when I tried to open by LO calc, it used to ask for the password, and after entering it, the file had been opened correctly.
From this month on, when I try to open such a file, LO Calc, without any prompt, opens it, which is apparently encrypted. The only readable string in the file is the following:

2StrongEncryptionTransformh EncryptedPackage2StrongEncryptionDataSpaceXL{FF9A3F03-56EF-4613-BDD5-5A41C1D07246}NMicrosoft.Container.EncryptionTransform<Microsoft.Container.DataSpaces$�$f��Microsoft Enhanced RSA and AES Cryptographic Provider

Can anyone know how to overcome the problem?

If it used to work in LibreOffice version x.x.x.x? and now doesn’t work in a current version (?) of LibreOffice then you should file a bug report, How to Report Bugs in LibreOffice - The Document Foundation Wiki. Cheers, Al

If it is really an encrypted with password document according to the OOXML specification then one simply can not open it without providing the proper credentials in some way. So either it is not, or the password is submitted automatically via some (system?) functionality or the document gets decrypted before LibreOffice even sees it when opening. But this is all speculation without the actual document at hand.

Do you mean, that the file opens in Writer, showing (mostly) garbage?

Oh that is indeed another possibility I didn’t even think of… raw encrypted content opened without decrypting it.

The question to you is: What has changed? LibreOffice or your files?
To test, try to open one of your older files. When you open them, are you asked for the password or not?
If you are asked for a password the format of the files may have changed. Load one of the older files in an Editor like Notepad++ and compare the beginning of the file or provide some samples.
If LibreOffice will not ask for passwords, where it did earlier, please tell the version of LibreOffice you use now and try to find out, wich version you used before. (Maybe in your folder for downloads you find all downloaded versions…). Providing a sample may help to verify a bug.
Bugs are reported then at Bugzilla… The link was already in the first comment above.

@mikeaganski Yes, exactly.

@Wanderer . Thank you. It is a good approach. The older files are working as they should. The problem is in files from another source. Please, pay attention to the string in the file. I guess they are using some encryption schema, that the LO does not recognize it.

This means: you found something not (yet) supported by LibreOffice (in the unknown version that you use) - and that means, that a user-to-user site for “how do I?” type of questions, as this Ask site is, is not a proper place to resolve this (except for a “test with a current version of LibreOffice first”). If you use (or tested that this is still a problem in) a current version (7.5 at this moment), then please file a bug report, with a sample problematic file attached.