Hi Kambala. I'm having the same problem.
I'm using ultimativeXVI and have used the provided zip.
I've made three characters to aid with debugging. The first character "Test" was created using the PlugY (version 11.02) client. The second character "TestNoPlugy" was created with the normal client. The third character "TestSomePlugy" was created using the normal client then played briefly using the PlugY client.
The Offline Tools fails to open "Test" and "TestSomePlugy", giving the error
Page 1 of '<name>.d2x' has wrong item header
, however "TestNoPlugy" is opened fine.
I am able to play any character with either client, however after playing a character even a single time using the PlugY client prevents it from being opened by Offline Tools, even if it was last played using the normal client.
Hope this helps. I can provide more information if needed.https://www.dropbox.com/sh/f4arqowdb254o82/AAAAq0P30XoLrLTcjTgTG9Hfa?dl=0