This one is DRMClien.dll, not px.dll (therefore not Sonic related)
You'll either need to remove support for Windows Media formats (WMA/WMV/ASF) by moving in_wm.dll out of the Winamp\Plugins folder, or upgrade to a newer version of Windows Media Player (WMP9.1 or newer needs to be installed so that Winamp can support WM formats).
There's also the possibility that it could be pmp_p4s.dll (the PlaysForSure Portable Player Support plugin) invoking the error, which requires WMP10 to be installed for it to function properly.
But you didn't include your List Of Plugins
, so I can't be sure exactly.
ps. Winamp 5.53 is the latest version (5.52 is no longer supported).