I have just updated from Solr 6.2.1 to 6.6.1. I put into place a fresh 6.6.1 and mounted our Core (umslogs). This loaded perfectly fine on port 8181 and our application is able to write/read data.
The problem started when I restart Solr 6.6.1 and the below error appeared after Solr 6.6.1 came up accessible via the web page.
null:org.apache.solr.core.SolrCoreInitializationException: SolrCore 'umslogs' is not available due to init failure: null *
Next my testing lead me to start up Solr on port 8282 that no application is connecting/reading/writing to. On this test umslogs core loads is perfectly fine after erroring above.
Next my testing lead me to close that writes/reads to Solr 8181umslogs core and shutdown Solr 8282 umslogs core. Then I restarted Solr back on Poret 8181 and the umslogs core loads properly and our application that that writes/reads to Solr 8181 is once again operational.
Our application has used Solr 4.10.x, then Solr 6.2.x okay. Then again I do not doubt that I might have done something wrong with the 6.6.1 upgrade that is causing the above behavior