Problem creating new bill

Hello,
Please can somebody guide me to overcome this issue.
I can't create bills. I get the following error message :
Erreur sur submitForm('../tool/saveObject.php', 'resultDiv', 'objectForm') :
Le formulaire '${3}' n'est pas un 'widget'.

I try to reinstall everything and i've upgraded to V6.4.3
Thank you in advance

Thank you for your reply.

here is the error in the Browser Console :
TypeError: mySelect is undefined projeqtorDialog.js:5009:3
TelemetryStopwatch: requesting elapsed time for nonexisting stopwatch. Histogram: "PLACES_AUTOCOMPLETE_1ST_RESULT_TIME_MS", key: "null" TelemetryStopwatch.jsm:363
timeElapsed resource://gre/modules/TelemetryStopwatch.jsm:363:9
finish resource://gre/modules/TelemetryStopwatch.jsm:382:17
finish resource://gre/modules/TelemetryStopwatch.jsm:225:12
_onResultRow jar:file:///usr/lib/firefox/omni.ja!/components/UnifiedComplete.js:1709:7
_onResultRow self-hosted:947:17
handleResult

And this is what I found in the log file : 2017-11-03 14:58:20.567 ***** ERROR ***** [V6.4.3] on file '/var/www/html/projeqtor/model/BillMain.php' at line (411)
2017-11-03 14:58:20.567 ***** ERROR ***** [V6.4.3] cause = A non well formed numeric value encountered
2017-11-03 15:04:27.464 ***** ERROR ***** [V6.4.3] ERROR *****
2017-11-03 15:04:27.464 ***** ERROR ***** [V6.4.3] on file '/var/www/html/projeqtor/model/BillMain.php' at line (411)
2017-11-03 15:04:27.464 ***** ERROR ***** [V6.4.3] cause = A non well formed numeric value encountered

I'm using projeqtor on Ubuntu 17.10 only on my laptop server. I'm testing the tools before using it for real work.
Thanks for your Help

What is your php version?

7.1

Maybe error of php 7.1 ... Try in PHP 5.6 or 7.0 please.