It can also be stale .elc files or something went wrong with loading new files into the running Emacs if you updated packages after upgrading to 26.1.
You can use FIND-FUNCTION to see the file Emacs last loaded a function from. If you set LOAD-PREFER-NEWER to T in your init file before loading any packages, Emacs will load the source file if the corresponding byte-compiled file is older.
Deleting the SLIME .elc files, re-compiling the .el files (* . elc D then * . el B in dired), and reloading Emacs will ensure that everything is loaded correctly.
Vladimir