Might you have checked the "Raw" button on the logging configuration page? That logs the raw data from the MUD including ANSI colour codes, which may look like "various symbols".
Unless you have a particular reason to, leave that unchecked.
Also, if you edit the log files do you actually see "畂瑲湯" or do you see the "chinese" characters, and they just look like that on the forum?
However if you aren't using it I wouldn't necessarily start now. I am more interested to getting to the bottom of what is happening.
If you want to try, a low-sequence trigger, matching everything, and with "keep evaluating" checked, could use a small bit of script to send lines to the log file.
I tried logging and it worked fine for me. Can you confirm a few other things?
Did you install other things recently? For example, Chinese fonts.
Have you started using encryption?
Did you set up some sort of automatic zip-up of your log files to save space?
What version of Windows do you have (or is it Ubuntu?)
Can you run a disk-check utility (eg. chkdsk or whatever it is called)? It is possible your disk is corrupted and you are reading back bad data.
Can you revert back to a version of MUSHclient that works? The Announcements section of the forum will have links to lots of older versions. If you can confirm that it works in version X, but not in X+1 that will help track it down.
Can you possibly try it on a different PC (eg, a laptop) to confirm if it is just confined to a particular PC?