If your email client displays an '-ERR Mailbox Locked' or '-ERR [IN-USE] Mailbox Locked' error when trying to log into VPOP3 it means that the mailbox is locked.
The POP3 standard requires that POP3 mailboxes have an 'exclusive lock' on them when they are in use. This means that only one POP3 client can access the mailbox at once. Also, if a POP3 mailbox is currently accessing a mailbox, then no IMAP4, Webmail or administrator access is allowed to the mailbox at the same time.
If you require two (or more) programs or users to access a mailbox at once, then you should consider using IMAP4 (with VPOP3 Enterprise) instead of POP3. IMAP4 is designed to allow multiple access to the same mailbox at the same time. POP3 specifically prohibits that behaviour.
If you think that only one thing is accessing the mailbox at once, then the simplest thing to do is to change the account password, and don't tell anyone, but just put into the one known program yourself. Then you know that no other program has the correct password, and no one else will put it into another program (or phone etc) without telling you.
Note that POP3 and IMAP4 access the same mailbox and both need and acquire locks on it. POP3 uses an exclusive lock and IMAP4 uses a shared lock. If something is accessing the mailbox using POP3, then nothing else can access the mailbox at the same time, using POP3 or IMAP4. If something is accessing the mailbox using IMAP4, then nothing can access the mailbox at the same time using POP3 (but it can using IMAP4). (Webmail has the same rules as IMAP4).
Note that the 'Mailbox locked' is not related to the account security lock which happens if someone tries to log in with an incorrect password too many times. The account lock will not be told to the user at all, but is just reported to them as an invalid login, to avoid giving a potential intruder too much information.