Chinese characters in mails vanish - PANIC!!!

Hi,
trying to switch from good old Outlook to Thunderbird Portable worked with just a few frictions. The import of quite a large amount of mails was possible only with tricks. So yesterday - all seemed to be fine. I was happy to find all my mails in TBP, I had the feeling to have mastered all the character encoding settings. TBP even shows Chinese characters in the subject line (that Outlook doesn't do).

Until today:
All the Chinese characters in my mails are gone and substituted by question marks.
Per-ma-nent-ly, as it seems.

I had all fonts and encodings for Chinese simplified defined in the options and that worked quite well yesterday. Today nothing but ????. I tried all possible encodings from the VIEW menu - nothing worked and brought back again the Chinese characters in the mails. Only in the subject lines Chinese characters are readable.

The original mails in Outlook still show the Chinese characters. My system (XP pro MUI, English with additional language packs) works still fine even with Chinese software. Saving files with eg Chinese or Russian characters in the file name is possible without problems. So I don't think it is a matter of settings in my system, but a problem of TBP.

I made another test by forwarding original mails with Chinese characters via Outlook and getting them again in TBP. The mails still show the Chinese characters as they should. But all other mail previously received in TBP seem to be damaged.

I'd be really grateful to get some tips how to store Chinese mails in TBP without having removed Chinese characters by TBP...