Belgium has a ban on cash transactions with “basic” bank accounts. At the same time, the EU Payment Services Directive (PSD) says:

Article 35
Access to payment systems

1. …
   Payment systems shall not impose on payment service providers, on payment service users or on other payment systems
   any of the following requirements:

(a) restrictive rule on effective participation in other payment systems;

Superficially, a cash ban would seem to block participation with another payment system. Of course the definition of “payment system” is relevant:

(7) ‘payment system’ means a funds transfer system with formal and standardised arrangements and common rules for
the processing, clearing and/or settlement of payment transactions;

Cash is a payment system of the central bank. Does cash meet that definition?

Update: Belgium’s transposition of the law explicitly excludes cash as a ‘payment system’ :

Royal Decree of 11 MARCH 2018
Annex I.B.
The following shall not be considered as payment services within the meaning of this Law:

1. The execution of payment transactions exclusively in cash and going directly from the payer to the beneficiary, without the intervention of any intermediary

(^ machine translated)