  The magnetic phase diagram of the chiral magnet Cu$_2$OSeO$_3$ has been
investigated by dc magnetisation and ac susceptibility over a very large
frequency range from 0.1 Hz up to 1 kHz and temperatures below 50 K.
Qualitatively different phase diagrams have been obtained by applying the
magnetic field along the easy $\langle100\rangle$ or the hard
$\langle110\rangle$ crystallographic directions. Strong $\chi"$ appears close
to $B_{c2}$, but only below 30 K and is more pronounced when $B$ is applied
along $\langle100\rangle$. In addition, the transition from the helical to the
conical phase leads to two adjacent $B_{c1}$ lines below 50 K. The frequency
dependence of $\chi"$ indicates the existence of very broad distributions of
relaxation times both at $B_{c1}$ and $B_{c2}$. The associated very long
characteristic times eventually prevent the system from reaching thermal
equilibrium at low temperatures and lead to thermal hysteresis.
