Searching Google for 'Plat 46' i found the following stuff on the Debian website (via google cache... http://22.214.171.124/search?q=cache:P1Yutlv9BWgJ:www3.symbian.com/faq.nsf
but this may not hold good long-term as a link as the cache will change in time).
<Extracts from Debian site>:-
Thus the reported plat:46 means ESystemException. The problem will very likely be resolved by restarting the app or the emulator.
These alert boxes shouldn't happen - they are typically associated with unexpected errors from underlying Win32 APIs, or some other indication that the emulator has got itself into a state which makes no sense in terms of EPOC: for example the EGuiStandbyWhilstOff message which happens if the emulated EPOC system is switched off while it is switched off (?!); or a needed dll is in use (being rebuilt?) when called.
</End of extracts from Debian website>
The debian page is not directly related to Psion 3 series, but there's a good chance that their error-code enumerations hold good for the message that you're seeing.
So - putting this back into the context of your failing Psion3mx connection attempt - it is likely an issue between the PsiWin software and the Win OS dll's that it is using.
Assuming that you used to be able to connect your 3mx and this problem has started happening recently. I also assume that you have already tried closing PsiWin, restarting your PC and trying again (if you haven't then try that). If you've already tried shutdown and restart and you still get the problem each time then perhaps the PsiWin software on PC needs reinstalling. Maybe a .DLL file got corrupted or deleted somehow.
I suggest you make backup copies of any data from your 3mx that is stored on your PC first though, if you decide to try reinstalling PsiWin.
As I don't know your level of experience with your Psion 3mx and PC, I am posting this purely to try and help 'shed some light' on the problem as you request. I have not had this problem with my Psion so have no 'tried and tested' answer.
Any decision on if and how to proceed in the light of this missive i've posted (and therefore any consequence of any decision) must be down to you.
Hope this helps you to figure out your connection problem and get your psion 3mx working with your pc again.