The extension (.DAT) is wrong for it being a chat log file of Messenger Plus!. Messenger Plus! chat log files which are encrypted should have the .PLE extension.
Ofcourse an extension on its own doesn't say much; the actual contents of the file is what matters. Encrypted Messenger Plus! log files always start with a file header starting with the ID string: "
MPLE".
To check this, open the files within an hex-editor or even notepad*. The very first characters should be "MPLE" if they realy are encrypted Messenger Plus! log files.
*Make sure you do not change anything in the file or save the files again. This to avoid damaging the contents.
eg:
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If they are indeed encrypted Messenger Plus! log files, you can rename the extension to PLE and try to open them with the Log Viewer of Messenger Plus!.
If you have Messenger Plus! installed you can simple double click the file, then the log viewer will be started automatically. Ofcourse you need to provide the password which they were encrypted with. If you don't know the password anymore then there is no way to decrypt the files.
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If they aren't encrypted Messenger Plus! log files then there is no easy way to know what they actually might be without seeing the complete contents of the files.