Transfer of larges files fails on forwarded ports

[Michael Wiedmann]

Given the following scenario (simplified, port no. examples only,
omitted auth):

User                                            embedded system
port 5000     -L ...    port 50000    -R ...    port 80
ssh client <----------> SSH Server <----------> ssh client
(plink/Win)          (dropbear/cygwin)          (dbclient)

Workflow is like:

- ssh client on right side (embedded system) initiates ssh session to
  dropbear ssh server requesting _remote_ port forwarding
- ssh client on left side (PC) initiates ssh session to the same
  dropbear ssh server requesting _local_ port forwarding

- User on the left side can point his web browser to localhost:5000
  and reaches via ssh tunnel and the forwarded ports the web server 
  on the embedded device (port 80)

- this works so far for "simple" web pages:
  User can browse configuration pages on the embedded device,
  make changes, etc.

BUT

if we try to transfer a "large" file via HTTP upload over the
tunnel/forwarded port
the transfer fails at same point (around 500 kB, varying).

If we do the same using only one ssh connection (between server and
embedded system) and try to transfer the same large file (using
server:50000) everything seems to work (at least we didn't notice any
failure on several tries). 

We suspect a socket read/write error but not even can see a real failure
message in the log file (we enabled dropbear TRACE).

Any hints how to further localize the observed error?

Thanks
Michael