I propose that you get 2.7.8 working. If the authentication fails, try do do password recovery, or recreated the accounts. 

You just start writing on top of the cited text 

 Thank you very much for your answer.

Is there any way to invert the order of the original text and the reply text ?

In principle you can adjust the number of lines shown in the summary text file with the option summary lines = x. If you increase that value, you start seeing some part of the reply. For elog itself  it is impossible to distinguish the reply text from the origial text, since some people use plain text like

with the reply sometimes below, sometimes above the original text. Other people use full HTML with tables (like in this forum), where it is very hard internally to distinguish between original and reply text. The easiest is not to quote the original text in replies, then you will see it always.

It is possible to show the (new) text of a replied entry in the summary text field ?

Actually only for the original entries the text was shown in the text column of the summary report, not for the replies.

I am using eLog ELOG V2.6.1-1681 which has 7 log books under 8 categories. Out of 7 log books, 2 are daily used. It happened that two weeks back. I modified information on two log books which were not used for quite longtime. Hence using CONFIG, I updated these log books, changing text etc. After that eLog in general started giving error. Any time when we click SUBMIT button in any log book, eLog shows page not found. That submit crashes eLog and as a result elogd service stops. Restarting elogd service, eLog operation comes back and the log which I submitted was actualy saved. Difficulty is that this problem is happening with every single submit action. 

I searched forum and learnt that upgrading eLog to newest version 2.7.8 will solve this problem. Upgrade created more problems, I was not able to open any log, authentication was not accepted. I restored that backup and went back to previous version. eLog started working but with submit error.

If any one can help me in fixing this problem

Just to say that this bug was fixed some time ago - I don't remember when (a senior moment).  
I know because I reported it to Stefan (along with the rest of the universe), as it was appearing 
on this forum when it was running on that particular SVN version. 

If you update to the latest version of elog, all will be fine.

David.
(helping Stefan, since I usually give him such a hard time).

When replying on a reply, the author is set to the author on the original reply? Othervice it workes great!

part of code:
...
Preset Author = $long_name
Locked Attributes = Author
Subst on reply author = $long_name
...