It would only happen with TCMD and IDE; TCC doesn't use scilexer.dll.
The strange thing is that the message is coming from the Windows loader, not TCMD, and it should cause TCMD to fail to load at all (scilexer.dll is a required dll). Scintilla is used for the command input window in TCMD -- can you access that window? Can you access the IDE (ide.exe or BDEBUGGER) at all? Is scilexer.dll in your installation directory?
I never use the command input window, and I have it hidden. I made it visible, and, sure enough, I cannot use it.
Yup, SCILEXER.DLL is not in my TC directory.
I will now reinstall and choose the repair option, as Stephen Worthington did, and report back as to whether the problem disappears.
(5 minutes later)
I reinstalled 14.0.25 and scilexer.dll is now present, and the error has disappeared.