@Gorki I would not use PPTP since it is not secure. Less secure than L2TP IPsec and OpenVPN. Rule this out.

<L2TP>

• What I've tested is that L2TP, iPhone works perfectly with the built-in VPN. Don't own an Andriod. You should check out the information you filled in on your Andriod devices. 
• Windows, indeed, fixed that with another patch. Luckily I did some research when this happened. Proved my thoughts, not an issue with the TP device. Do you still have errors on your Window PCs? 

About error 651, what I found for you, you can give it another try: https://superuser.com/questions/303389/connecting-to-vpn-i-get-error-651-the-modem-or-other-connecting-device-has

<OpenVPN>

For the OpenVPN username and password, I've searched this on Google. Seems that you can access the file and configure that? You can try that. Not sure if they'll add your suggestion in the roadmap. 

@John1234 thx 4 quick feedback!

kindly find my update:

<L2TP> 

Android:    after deleting + reconfiguring L2TP Setup on Smartphone + Omada, it worked fine now!

Windows:  thx 4 the superuser-link ... i went through all steps, removed suspicious hidden devices etc., but get still the same error ...

                 i will take another windows box + retry with this tomorrow...

<OpenVPN>

The *.ovpn File created by Omada can for sure be edited.
a sample file can be checked at: https://github.com/OpenVPN/openvpn/blob/master/sample/sample-config-files/client.conf

.) For example the "cipher" entry can be changed (for sure Omada Router needs to support it also ...)

    cipher AES-256-GCM

.) or the "remote" entry to make use of a fqdn instead of a ip-address.

    remote myhostname.aaa.bbb 65000

regarding OpenVPN username and password,

from my pov, it should be confiurable on Omada side,

otherwise Router has no information which user + password credentials are correct.

Without this feature, everyone who has access to the *.ovpn File

can Access the VPN, without further authentication...

Cheers Gorki

@Gorki 

Could be something added in the future. Rumor says roadmap V5.2 will have many changes on OpenVPN. 

I searched OpenVPN password edit in the notepad. Seems to be editable even the controller does not support username and password. I don't know about that and am less worried about this. This could be a concern to someone else. 

@Gorki 

<L2TP>

L2TP with PSK on a different windows box (fresh installed + patched) worked fine.

=> OpenVpn + L2TP are connectable now.

Ill proceed with further functionality testing this week