When I save an .xlsx with a password, the password doesn’t work upon open. After about 20 tries later, I can confirm there are definitely no typos, no caps lock, no num lock, etc. This seems to be a problem with LibreOffice calc. Could it be because my password is 21 characters long? Using a single character like “a” as the password works just fine.
Yes, there is a password length limit applied by LO for all MS formats, binary (tdf#65492) and OOXML (tdf#97086). The former is not related to your question (there is a problem of opening non-conforming binary documents created by a specific MS Office version), but the latter is the bug affecting you. LO only uses first 15 characters of entered password to encrypt the document; so on opening time, you need to only enter 15 initial characters (LO doesn’t truncate the password on opening).
Thank you!
Please, if the answer solves the question click 
.
Happy to. Where exactly do I do that? On mobile.
Under the answer votes.