Undefined Apache server error rdbmmap -

since few days i've got following error message in server error log :

www.xxx.com [client xxx.xxx.xxx.xxx] rdbmmap (21): rdbm_fetch failed (1)

although site loading right. know error. can't find around. know this?

server configuration :

server-info: apache/2.2.31 (unix)
php-version: 7.1.8
wordpress: 4.8.1
mysql-version: 5.6.37

thanks in advance.