I am using DSRP and AEM 6.2.
I fixed the issue.
The issue is in class "com.adobe.cq.social.datastore.rdb.impl.DSRPUtil.java" that belongs to JAR "cq-social-rdb-provider-1.1.19.jar".
In "populateBlob" method, I replaced some part of code with below line:
String cqdata = trimNull(IOUtils.toString(blobStream, "UTF-8"));
and it worked.