This is about a new error message in convert/piecewise not caught in time within the internal routines after having computed a solution (for the piecewise form, not the Heaviside form, which requires an optional argument with a method). It is fixed, and the fix is distributed to everybody using Maple 2022 within the Maplesoft Physics Updates v.1257.
PS1: there are other methods that solve the same equation, it is an easy equation; try for instance dsolve(de, [exact]).
PS2: if you find something unclear on the page ?dsolve,parametric, please post the issue, thanks; I will then give it a look.
Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft