I'm while using solr-for-wordpress wordpress plugin, however when i run indexing all posts, i get a mistake like:

"400" Status: Illegal character ((Control-CHAR, code 7)) at [row,col ]: [38878,5]

anybody knows why?

This can be a common problem because of certain figures inside your content requiring to become correctly steered clear of prior to being posted in XML format towards the server.

Here's a good example of how one handled this need in Java:

Update with Invalid Control Character Crashes Solr