I have an extension and a frontend form to create and update elements. The user can create an invoice with several dates which are stored as m:n records.

Creating an invoice and dates works with:

```
<f:form.checkbox name="newRechnungen[termine][]" value="{termin.uid}" />

```

But if I edit invoice and submit form, I get errors:

```
<f:form.checkbox name="rechnungen[termine][]" value="{termin.uid}" />

```

shows errror like:

> 
> The form field "[items][][]" is invalid. Reason: "[]" used not as last argument, but somewhere in the middle (like foo[][bar])
> 
> 
> 

changing the tag like described for solution to 

```
<f:form.checkbox name="rechnungen[termine][{key}]" value="{termin.uid}" />

```

shows error:

> 
> #1297759968: Exception while property mapping at property path "files": PHP Warning: spl\_object\_hash() expects parameter 1 to be object, null given in /usr/share/typo3/typo3\_src-7.6.2/typo3/sysext/extbase/Classes/Persistence/ObjectStorage.php line 155 =
> 
> 
> 

The solution described on the documentation page with `enctype="multipart/form-data"` for form tag is not working.

Any idea how I can solve this?