  We present 36 new mid-eclipse times obtained between 2017 and 2023 using the
T100 telescope in Turkey, extending the time span of the $O-C$ diagram to 25
years. Once again, these new observations show significant deviations from
previous published models that were able to explain the observed variations of
the binary period. We investigate two plausible explanations for this
variability: the LTT effect due to the presence of one or two invisible
low-mass (planetary) companion(s) in distant circumbinary orbits; other
mechanisms, like e.g. the Applegate mechanism, associated with the magnetic
cycles of the M-dwarf component of the WD+dM binary. Through MCMC analyzes, we
demonstrate that the observed $O-C$ variability can be explained by the
presence of a planet with a minimum mass of $\sim9.5 M_J$. This circumbinary
planet orbits around the binary system with a period of about 19.5 years,
maintaining a stable orbit for a timeline of 10 Myr. By adding a weak LTT
signal from a secondary hypothetical planet we achieve statistically better
results. However, the orbits of the bodies in a two-planet system remain stable
only for a small range of the parameter space. The energy required to power the
Applegate and other Applegate-like mechanisms is too high to explain the period
variations observed. Thus, on the one hand there is substantial evidence
supporting the existence of a planet in the NN Ser system, but on the other
hand there are also compelling indications that cast doubt on the existence of
a second hypothetical planet.
