Symex uses advice to implement some features such as adding a selection overlay. To minimize complexity while debugging, it may be advisable (so to speak) in certain cases to disable such advice. To do this, find the place in the code where the advice is added, and execute the corresponding function to remove it. Something like, (advice-remove #'symex-user-select-nearest #'symex-mode-highlight-selected). Of course, if disabling the advice causes the error to go away, then you can focus your efforts on debugging the advice itself in isolation!
It may also be advisable to comment out macros like symex-save-excursion to see if the problem persists. Commenting out macros like symex--with-undo-collapse will also help you use the debugger in code wrapped by such macros.
Also See Troubleshooting.