I recently installed CyanogenMod on my Galaxy Ace and now its USB tether option is not working. The phone is detected by my PC only when I check the USB debugging mode. I can tether with external applications like Easy Tether but earlier USB tethering was built in for my phone when I had the Samsung stock firmware. What could be the possible problem?
ROMs like CyanogenMod and others by no means are complete. They are always under major refactoring and between minor revisions major functionality may be broken. For a long time I have been using custom roms, and if some functionality is broken I would advise you to return to a previous version of the ROM, or switch to another ROM.
The problem can also be related to the kernel you are using. Kernel is different than the ROM. ROM is built on top of kernel meaning that kernel part covers the more low-level functionality. If the kernel you are using does not support some functionality about USB bus, then you can still have problems although the ROM has no bugs.