Problem with uploading files when traffic goes through opnsense 16.7.3

[DoubleJ]

Hi, I encounter a problem and can't figure out what the problem is.

I have a opnsense 16.7.3 installation running as router on my local lan.
My LAN has two gateways:
- one gateway is on the mentioned opnsense box (ip: 10.0.0.5/24) -> WAN interface connected to another DMZ network.
- one gateway is another router (not opnsense) on the LAN (10.0.0.1/24), which is directly connected to the internet.
All clients have the opnsense box set as default gateway. The purpose of the opnsense box is to split up the traffic bound for the internet (routes the traffic to the other gateway on the LAN: 10.0.0.1 gateway) and the traffic bound for the DMZ (routes it over the WAN). The splitting up is done with the help of static routes.
NAT rules are only applied on the WAN outbound, source = LAN and destination = any. Of course no NAT rules on the LAN interface. Firewall rules on LAN: any to any allowed on any protocol. Firewall rules on WAN: source = LAN to any allowed on any protocol.

Problem: web browsing is no problem, however if I want to upload an attachment to a webmail account on the internet I can't upload the attachment. If I try to send files over the internet to a git server (by use of http/https protocol) I can't upload the files; Git reports error: RPC failed; result=56, HTTP code = 0.
When I do a tracert from the client to the git server on the internet, I can see that the traffic is routed from opnsense to the other gateway on the LAN and doesn't show any problem.
The problem is very likely with opnsense, because if I change the default gateway on the client directly to the other gateway (so bypassing the opnsense router) everything works; I can upload attachments to my webmail accounts and upload my files to the git server without any problem.

I have not set any special settings on opnsense other than static routes, no proxy modules enabled.

Does anyone have an idea why I can browse the web, but not able to upload files?

Thanks in advance.
DJ