I want to share some other discovery I made today:

In the last few days I was playing around with several other settings like --queue --expiry and --scan-time. And the more time passed, the less blocks my miners picked up. I tried all kinds of things to get it working again like resetting router, restarting rigs, ... I was about to give up when it occurred to me that the part of these settings might still have been stored and corrupted my mining settings. Then I remembered something you do when testing out new settings, to delete the "scrypt130511Tahitiglg2tc8192w256l4.bin" file in the cgminer folder.

No more than 10 minutes passed after deleting this file and my miners found already the first block again.

To make a long story short - if you play around with several settings and experience bad luck, try deleting this file - as this will reset and rebuild your proper mining settings.

Also, another tip:

If you add at least 2 --fallover-only -o stratum+tcp://pool.com:xxxx -u user -p password backup pools to your .bat file, you will increase your chances of finding a block while solo mining, as you will use the stratum's servers data to update your miner when a block gets found, so you are always working on the latest block. (And thus avoiding orphaned blocks.)