Dear Scott,
Thank you very much for your help. The error (that should be related with BIRT) is thrown directly at startup after I launch KNIME, but I cannot see anything strange in the log file.
Here you have the log file when KNIME throws the error starting from a fresh installation of KNIME 3.7.1 after installing some extensions:
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : #########################################################################################
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # Welcome to KNIME Analytics Platform v3.7.1.v201901281154 (Build January 29, 2019) #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # Based on Eclipse, http://www.eclipse.org #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : #########################################################################################
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # Copyright by KNIME AG, Zurich, Switzerland and others. #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # Website: http://www.knime.com #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # E-mail: contact@knime.com #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : #########################################################################################
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # For more details see the KNIME log file: #
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : # /home/giovanni/workspaces/knime/.metadata/knime/knime.log
2019-02-26 09:28:36,165 : INFO : main : NodeLogger : : : #---------------------------------------------------------------------------------------#
2019-02-26 09:28:36,166 : INFO : main : NodeLogger : : : # logging date=Tue Feb 26 09:28:36 CET 2019
2019-02-26 09:28:36,166 : INFO : main : NodeLogger : : : # java.version=1.8.0_152
2019-02-26 09:28:36,166 : INFO : main : NodeLogger : : : # java.vm.version=25.152-b16
2019-02-26 09:28:36,166 : INFO : main : NodeLogger : : : # java.vendor=Oracle Corporation
2019-02-26 09:28:36,166 : INFO : main : NodeLogger : : : # os.name=Linux
2019-02-26 09:28:36,166 : INFO : main : NodeLogger : : : # os.arch=amd64
2019-02-26 09:28:36,166 : INFO : main : NodeLogger : : : # number of CPUs=8
2019-02-26 09:28:36,166 : INFO : main : NodeLogger : : : # assertions=off
2019-02-26 09:28:36,167 : INFO : main : NodeLogger : : : # host=woodstock
2019-02-26 09:28:36,168 : INFO : main : NodeLogger : : : # username=giovanni
2019-02-26 09:28:36,168 : INFO : main : NodeLogger : : : # max mem=1820MB
2019-02-26 09:28:36,168 : INFO : main : NodeLogger : : : # application=org.knime.product.KNIME_APPLICATION
2019-02-26 09:28:36,169 : INFO : main : NodeLogger : : : # ID=01-cef90b5a0e28061d
2019-02-26 09:28:36,169 : INFO : main : NodeLogger : : : #########################################################################################
2019-02-26 09:28:36,169 : WARN : main : ProfileManager : : : Could not write combined preferences file '/home/giovanni/workspaces/knime/.metadata/.plugins/org.knime.product/combined-preferences.epf', will use temporary file '/tmp/combined-preferences7158697349382813644.epf' instead.
2019-02-26 09:28:42,212 : DEBUG : main : DatabaseConnectionSettings : : : Settings database timeout to 15 seconds
2019-02-26 09:28:43,107 : DEBUG : main : DatabaseConnectionSettings : : : Database concurrency (sync via database connection) is true.
2019-02-26 09:28:43,108 : DEBUG : main : KNIMECorePlugin : : : Setting KNIME max thread count to 8
2019-02-26 09:28:43,108 : DEBUG : main : KNIMECorePlugin : : : Setting KNIME temp dir to "/tmp"
2019-02-26 09:29:08,261 : ERROR : Worker-1 : SparkProviderRegistry : : : Problems during initialization of Spark provider with id 'org.knime.bigdata.spark.local.SparkLocalNodeFactoryProvider'. Exception: Plug-in org.knime.bigdata.spark.local was unable to load class org.knime.bigdata.spark.local.SparkLocalNodeFactoryProvider.
2019-02-26 09:29:08,277 : ERROR : Worker-1 : SparkProviderRegistry : : : Extension org.knime.bigdata.spark.local ignored.
2019-02-26 09:29:13,837 : WARN : Worker-1 : Node : Create File Name : 1:5:9 : Selected directory '/tmp/knime_tc_2u4yohvcb9gw' cannot be accessed!
2019-02-26 09:29:13,952 : WARN : Worker-1 : Node : Create File Name : 1:6:9 : Selected directory '/tmp/knime_tc_mbamo8mnif2i' cannot be accessed!
2019-02-26 09:29:14,103 : WARN : Worker-1 : Node : Create File Name : 1:7:9 : Selected directory '/tmp/knime_tc_4lr8relatpaq' cannot be accessed!
2019-02-26 09:29:14,208 : WARN : Worker-1 : Node : Create File Name : 1:8:9 : Selected directory '/tmp/knime_tc_0geku89fa42c' cannot be accessed!
2019-02-26 09:29:14,329 : WARN : Worker-1 : Node : Create File Name : 1:10:9 : Selected directory '/tmp/knime_tc_vr6gqqptdnis' cannot be accessed!
2019-02-26 09:29:14,433 : WARN : Worker-1 : Node : Create File Name : 1:11:9 : Selected directory '/tmp/knime_tc_rcsibqomys15' cannot be accessed!
Does this help? Do you have any suggestion?