[TYPO3-Solr] Can't commit pending documents to SOLR-Server

[Gordon Fidora]

Hi,

generally I get a connection to our SOLR-Server. The initial Task "Index-Queue Worker" runs successfully and is committing data to SOLR-Server via POST.

But after, I can't update and commit pages to our SOLR-Server. If I try updating manually in T3-Backend via "Search->Commit Pending Documents" I got in Developer LOG an 'Apache_Solr_HttpTransportException' with message ''503' Status: Service Unavailable.
This error occurs on querying SOLR data via POST.

Before I tried the "Commit-Task" unsuccessfully. But I read not to use this Task anymore.
I got this message: Commit Solr Index (solr)" failed with the following message: Solr response does not appear to be valid JSON, please examine the raw response with getRawResponse() method.

My System:
TYPO3 6.1.5
SOLR-Extension 2.8.3
SOLR Server installed on another System (Debian Linux) with own IP-Address
Tomcat Installation came from SOLR-Extension Install-Script (Problem also occurs with Debian-Tomcat).

Thx in advance

Gordon