BC365:error when designing page (on-Premise install)

hi,
I'm stuck on an BC365 Wave 2 (15) environment. My SQL is on Azure, my Administration Tier is on an Azure VM.
BC works fine. The database is a Migrated one (NAV16 ->BC13 and now BC15)
In the webclient, on any pages, in Designer mode, as soon as I drop a field, error is raising "something went wrong".

The EventLog of the server says:

• Category: DatabaseSync: Message (NullReferenceException): RootException: NullReferenceException
  
• Object reference not set to an instance of an object.
• Category:Extensions: Message: same as above
• Category : Development: Message: Message Failed to save the extension. Object reference not set to an instance of an object.
  
• Category: Extensions : Message (NavDesignerExtensionStorageException): RootException: NavDesignerExtensionStorageException
• Category: Extensions: Message The execution of the code block at HandleAddPageFieldAction had one or more unhandled exceptions., Additional info: NSDesigner

and some more ..
EventLogWriter
Unhandled error occurred on a browser or device client:
{
"clientInfo": {
"platform": "Win32",
"userAgent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:70.0) Gecko/20100101 Firefox/70.0",
"browserLanguage": "fr"
},
"message": "TypeError: $0.setCurrenRow is not a function",
"pageUrl": "https://theurlofmyBVinstance",
"originalError": {},
"location": {
"name": "https://theurlofmybcinstance/js/framework.js?h=888D73829F5FCAB0537812DF9B7F596A9042E25F67D6044B18BC0A82AD2723A3&_v=15.0.36510.0",
"url": "https://theurlofmyBCinstance/js/framework.js?h=888D73829F5FCAB0537812DF9B7F596A9042E25F67D6044B18BC0A82AD2723A3&_v=15.0.36510.0",
"lineNumber": 913,
"columnNumber": 55
},
"detailedMessage": "whenNotBusy/</

...then it lose the connection.
I hardly think it is linked to the following: I can't install the "Designer" Extension

When I try to install the "Designer_xyz V1.0.0.0" extension I got "Exception thrown during implicit sync of extension Designer_bca51682-0620-4803-bbd1-cde41742a9dd by default::myname : Object reference not set to an instance of an object."

How can I get it works? thanx volks, see you in Antwerpen.
Chris