ionic cordova platform add fails with local plugins

**Description:**
Hi there!
This happens since about 2 days. When i add a platform like ios with 'ionic cordova platform add ios', it fails, because i have local plugins added to my config.xml.
When i use cordova platform add ios instead, it works.
Am i supposed to only use cordova platform add?

**Steps to Reproduce:**
ionic start myApp tabs
(i copied a plugin to ProjectDir/local-plugins/PluginDir)
ionic cordova plugin add local-plugins/PluginDir
ionic cordova platform add ios -> fails
rm -rf platforms
rm -rf plugins
cordova platform add ios -> success

**Output:**
[ERROR] An error occurred while running subprocess cordova.
--verbose gives me no more infos

**My `ionic info`:**
Ionic:

ionic (Ionic CLI) : 4.1.2 (/usr/local/lib/node_modules/ionic)
Ionic Framework : ionic-angular 3.9.2
@ionic/app-scripts : 3.2.0

Cordova:

cordova (Cordova CLI) : 8.0.0
Cordova Platforms : ios 4.5.5
Cordova Plugins : cordova-plugin-ionic-keyboard 2.1.2, cordova-plugin-ionic-webview 2.1.4, (and 4 other plugins)

System:

Android SDK Tools : 26.1.1 (/Users/schoko/Library/Android/sdk)
ios-deploy : 2.0.0
ios-sim : 6.0.0
NodeJS : v8.12.0 (/usr/local/bin/node)
npm : 6.4.1
OS : macOS High Sierra
Xcode : Xcode 9.4.1 Build version 9F2000

**Other Information:**
I tried to roll back ionic, app-scripts ... but no change.