Neoliz

def post_list(request):
    clients = Client.objects.prefetch_related('contacts').all()
    return render(request, 'blog/post_list.html', {'clients': clients})

def post_list(request):
    clients = Client.objects.all().prefetch_related('contacts')
    return render(request, 'blog/post_list.html', {'clients': clients})

from django.db.models import Prefetch

def post_list(request):
    clients = Client.objects.all().prefetch_related(Prefetch('contacts', queryset=Contanct.objects.all()))  # В Queryset скармливаешь просто queryset и делается вложенный запрос.
    return render(request, 'blog/post_list.html', {'clients': clients})

Discussion.objects.all().annotate(
                comment_old_date=Max(
                    Case(
                        When(
                            comments__date_create__isnull=True,
                            then=datetime.datetime.strptime('1950-01-01 00:00:00', '%Y-%m-%d %H:%M:%S')
                        ),
                        default='comments__date_create'
                    )
                )
            ).order_by('-comment_old_date')

class UserCreationForm(forms.ModelForm):
    password1 = forms.CharField(
        label='passwd',
        widget=forms.PasswordInput(attrs={
            'style':'margin:10px; padding:10px;height:40px',
            'class':'form-control col-sm-8',
                'placeholder': 'Password please'
        })
    )
    password2 = forms.CharField(
        label='double passwd',
        widget=forms.PasswordInput(attrs={
            'style':'margin:10px; padding:10px;height:40px',
            'class':'form-control col-sm-8',
                'placeholder': 'Double password please'
        })
    )

    def clean_password2(self):
        password1 = self.cleaned_data.get('password1')
        password2 = self.cleaned_data.get('password2')
        if password1 and password2 and password1 != password2:
            raise forms.ValidationError('Passwd and double passwd error')
        return password2


    def save(self, commit=True):
        user = super(UserCreationForm, self).save(commit=False)
        user.set_password(self.cleaned_data['password1']) # важно тут передавать password1 или 2. Но не password как в примере выше
        if commit:
            user.save()
        return user
    

    class Meta:
        model = get_user_model()
        fields = ('email', 'lastname', 'firstname')
        widgets = {
            'email': forms.TextInput(attrs={
                'style':'margin:10px; padding:10px;height:40px',
                'class':'form-control col-sm-8',
                'placeholder': 'E-mail please'
            }),
            'lastname': forms.TextInput(attrs={
                'style':'margin:10px; padding:10px;height:40px',
                'class':'form-control col-sm-8',
                'placeholder': 'Lastname please'
            }),
            'firstname': forms.TextInput(attrs={
                'style':'margin:10px; padding:10px;height:40px',
                'class':'form-control col-sm-8',
                'placeholder': 'Firstname please'
            }),
        }

def registerPost(request):
    if request.method == "POST":
        user_form = UserCreationForm(request.POST)
        
        if user_form.is_valid():
            new_user = user_form.save(commit=False)
            new_user.set_password(user_form.cleaned_data['password1'])
            new_user.avatar = "fotos/no_image_dummy.png"
            new_user.save()
            return HttpResponseRedirect("/login")
    else:
        user_form = UserCreationForm()
    return render(request, 'register.html', {'form':user_form})

Скорее всего в ваших объектах есть несериализуемые типы. Попробуйте преобразовать через json.dumps(t) и отловить эксепшен.

class Tiket(models.Model):
    user = models.ForeignKey(User)
    name = models.CharField(max_length=350)
    info = models.TextField()
    sost = models.BooleanField(default=True)  # True - открыт тикет
    date = models.DateTimeField('Дата создания')
    manager = models.ForeignKey(Manager_cab)  # Прикреплен ли менеджер
    new_mess = models.BooleanField(default=False)
    kol_mess = models.IntegerField(default=0)