diff --git a/src/js/plugins/previewThumbnails.js b/src/js/plugins/previewThumbnails.js
index bd7a6bbd..813bc47e 100644
--- a/src/js/plugins/previewThumbnails.js
+++ b/src/js/plugins/previewThumbnails.js
@@ -17,17 +17,17 @@ const parseVtt = vttDataString => {
             if (!is.number(result.startTime)) {
                 // The line with start and end times on it is the first line of interest
                 const matchTimes = line.match(
-                    /([0-9]{2}):([0-9]{2}):([0-9]{2}).([0-9]{2,3})( ?--> ?)([0-9]{2}):([0-9]{2}):([0-9]{2}).([0-9]{2,3})/,
+                    /([0-9]{2})?:?([0-9]{2}):([0-9]{2}).([0-9]{2,3})( ?--> ?)([0-9]{2})?:?([0-9]{2}):([0-9]{2}).([0-9]{2,3})/,
                 ); // Note that this currently ignores caption formatting directives that are optionally on the end of this line - fine for non-captions VTT
 
                 if (matchTimes) {
                     result.startTime =
-                        Number(matchTimes[1]) * 60 * 60 +
+                        Number(matchTimes[1] || 0) * 60 * 60 +
                         Number(matchTimes[2]) * 60 +
                         Number(matchTimes[3]) +
                         Number(`0.${matchTimes[4]}`);
                     result.endTime =
-                        Number(matchTimes[6]) * 60 * 60 +
+                        Number(matchTimes[6] || 0) * 60 * 60 +
                         Number(matchTimes[7]) * 60 +
                         Number(matchTimes[8]) +
                         Number(`0.${matchTimes[9]}`);
@@ -148,7 +148,10 @@ class PreviewThumbnails {
 
                 // If the URLs don't start with '/', then we need to set their relative path to be the location of the VTT file
                 // If the URLs do start with '/', then they obviously don't need a prefix, so it will remain blank
-                if (!thumbnail.frames[0].text.startsWith('/')) {
+                // If the thumbnail URLs start with with none of '/', 'http://' or 'https://', then we need to set their relative path to be the location of the VTT file
+                if (!thumbnail.frames[0].text.startsWith('/') &&
+                    !thumbnail.frames[0].text.startsWith('http://') &&
+                    !thumbnail.frames[0].text.startsWith('https://')) {
                     thumbnail.urlPrefix = url.substring(0, url.lastIndexOf('/') + 1);
                 }