Ok, found it!
A quick fix described below.
But this should be fixed as soon as possible by the plugin owner.
The issue is wrong references to jQueryUI libs. I don’t know when this changed, but it seems that jQueryUI has left out the namespace in the js-files, e.g. jquery.ui.core.min.js has been renamed to core.min.js and so on.
I did a search on files and it seems to be enough to edit two files in CP Multi Event Calendar folder, where we have to fix the links:
Search "jquery.ui." (14 hits in 2 files)
\wp-content\plugins\cp-multi-view-calendar\cp-main-class.inc.php (9 hits)
Line 199: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.core.min.js'></script>
Line 200: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.widget.min.js'></script>
Line 201: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.mouse.min.js'></script>
Line 202: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.resizable.min.js'></script>
Line 203: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.draggable.min.js'></script>
Line 204: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.button.min.js'></script>
Line 205: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.position.min.js?'></script>
Line 206: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.dialog.min.js'></script>
Line 207: <script type='text/javascript' src='<?php echo $plugin_url.'/../../../'; ?>wp-includes/js/jquery/ui/jquery.ui.datepicker.min.js'></script>
\wp-content\plugins\cp-multi-view-calendar\php\edit.php (5 hits)
Line 58: <script type='text/javascript' src='<?php echo $path.'../../../../wp-includes/js/jquery/ui/jquery.ui.core.min.js'; ?>'></script>
Line 59: <script type='text/javascript' src='<?php echo $path.'../../../../wp-includes/js/jquery/ui/jquery.ui.widget.min.js'; ?>'></script>
Line 60: <script type='text/javascript' src='<?php echo $path.'../../../../wp-includes/js/jquery/ui/jquery.ui.button.min.js'; ?>'></script>
Line 61: <script type='text/javascript' src='<?php echo $path.'../../../../wp-includes/js/jquery/ui/jquery.ui.dialog.min.js'; ?>'></script>
Line 62: <script type='text/javascript' src='<?php echo $path.'../../../../wp-includes/js/jquery/ui/jquery.ui.datepicker.min.js'; ?>'></script>