diff --git a/Dockerfile b/Dockerfile
index 0cb0b38ca..a403fe2df 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM photoprism/development:20200113
+FROM photoprism/development:20200117
 
 # Set up project directory
 WORKDIR "/go/src/github.com/photoprism/photoprism"
diff --git a/Makefile b/Makefile
index a6aa0419f..ff863e9cc 100644
--- a/Makefile
+++ b/Makefile
@@ -137,8 +137,9 @@ lint-js:
 	(cd frontend &&	npm run lint)
 fmt-js:
 	(cd frontend &&	npm run fmt)
-fmt-go:
+fmt-imports:
 	goimports -w pkg internal cmd
+fmt-go:
 	go fmt ./pkg/... ./internal/... ./cmd/...
 tidy:
 	go mod tidy
diff --git a/docker/development/Dockerfile b/docker/development/Dockerfile
index b70e46069..a70f54d9e 100644
--- a/docker/development/Dockerfile
+++ b/docker/development/Dockerfile
@@ -51,7 +51,8 @@ RUN apt-get update && apt-get upgrade && \
     exiftool
 
 # Install RAW to JPEG converter
-RUN add-apt-repository ppa:pmjdebruijn/darktable-release && \
+RUN sh -c "echo 'deb http://download.opensuse.org/repositories/graphics:/darktable:/master/xUbuntu_18.04/ /' > /etc/apt/sources.list.d/graphics:darktable:master.list" && \
+    wget -qO - https://download.opensuse.org/repositories/graphics:darktable:master/xUbuntu_18.04/Release.key | apt-key add - && \
     apt-get update && \
     apt-get install darktable && \
     apt-get upgrade && \
diff --git a/docker/photoprism/Dockerfile b/docker/photoprism/Dockerfile
index 0fccb15e5..b19bbfc37 100644
--- a/docker/photoprism/Dockerfile
+++ b/docker/photoprism/Dockerfile
@@ -1,4 +1,4 @@
-FROM photoprism/development:20200113 as build
+FROM photoprism/development:20200117 as build
 
 # Set up project directory
 WORKDIR "/go/src/github.com/photoprism/photoprism"
@@ -27,12 +27,20 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     rm -rf /var/lib/apt/lists/*
 
 # Copy dependencies
-COPY --from=build /etc/apt/sources.list.d/pmjdebruijn-ubuntu-darktable-release-bionic.list /etc/apt/sources.list.d/pmjdebruijn-ubuntu-darktable-release-bionic.list
-COPY --from=build /etc/apt/trusted.gpg.d/pmjdebruijn_ubuntu_darktable-release.gpg /etc/apt/trusted.gpg.d/pmjdebruijn_ubuntu_darktable-release.gpg
 COPY --from=build /usr/lib/libtensorflow.so /usr/lib/libtensorflow.so
 COPY --from=build /usr/lib/libtensorflow_framework.so /usr/lib/libtensorflow_framework.so
 RUN ldconfig
 
+# Install RAW to JPEG converter
+RUN sh -c "echo 'deb http://download.opensuse.org/repositories/graphics:/darktable:/master/xUbuntu_18.04/ /' > /etc/apt/sources.list.d/graphics:darktable:master.list" && \
+    wget -qO - https://download.opensuse.org/repositories/graphics:darktable:master/xUbuntu_18.04/Release.key | apt-key add - && \
+    apt-get update && \
+    apt-get install darktable && \
+    apt-get upgrade && \
+    apt-get dist-upgrade && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
+
 # Install darktable (RAW to JPEG converter)
 RUN apt-get update && \
     apt-get install -y --no-install-recommends darktable && \
diff --git a/internal/photoprism/convert.go b/internal/photoprism/convert.go
index 8b19cbf9e..8e600ff6d 100644
--- a/internal/photoprism/convert.go
+++ b/internal/photoprism/convert.go
@@ -1,11 +1,14 @@
 package photoprism
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
+	"sync"
 
 	"github.com/photoprism/photoprism/internal/config"
 	"github.com/photoprism/photoprism/internal/event"
@@ -15,7 +18,8 @@ import (
 
 // Convert represents a converter that can convert RAW/HEIF images to JPEG.
 type Convert struct {
-	conf *config.Config
+	conf     *config.Config
+	cmdMutex sync.Mutex
 }
 
 // NewConvert returns a new converter and expects the config as argument.
@@ -140,11 +144,31 @@ func (c *Convert) ToJpeg(image *MediaFile) (*MediaFile, error) {
 		return NewMediaFile(jpegFilename)
 	}
 
-	if convertCommand, err := c.ConvertCommand(image, jpegFilename, xmpFilename); err != nil {
-		return nil, err
-	} else if err := convertCommand.Run(); err != nil {
+	cmd, err := c.ConvertCommand(image, jpegFilename, xmpFilename)
+
+	if err != nil {
 		return nil, err
 	}
 
+	// Unclear if this is really necessary here, but safe is safe.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	// Make sure only one command is executed at a time.
+	// See https://photo.stackexchange.com/questions/105969/darktable-cli-fails-because-of-locked-database-file
+	c.cmdMutex.Lock()
+	defer c.cmdMutex.Unlock()
+
+	// Fetch command output.
+	var out bytes.Buffer
+	var stderr bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &stderr
+
+	// Run convert command.
+	if err := cmd.Run(); err != nil {
+		return nil, errors.New(stderr.String())
+	}
+
 	return NewMediaFile(jpegFilename)
 }
diff --git a/internal/photoprism/import_worker.go b/internal/photoprism/import_worker.go
index 1281d4e78..92fa6b14d 100644
--- a/internal/photoprism/import_worker.go
+++ b/internal/photoprism/import_worker.go
@@ -66,7 +66,7 @@ func importWorker(jobs <-chan ImportJob) {
 
 			if importedMainFile.IsRaw() || importedMainFile.IsHEIF() || importedMainFile.IsImageOther() {
 				if _, err := imp.convert.ToJpeg(importedMainFile); err != nil {
-					log.Errorf("import: could not create jpeg from \"%s\"", err)
+					log.Errorf("import: creating jpeg failed (%s)", err.Error())
 				}
 			}
 
@@ -74,7 +74,7 @@ func importWorker(jobs <-chan ImportJob) {
 				log.Error(err)
 			} else {
 				if err := jpg.RenderDefaultThumbnails(imp.conf.ThumbnailsPath(), false); err != nil {
-					log.Errorf("import: could not create default thumbnails (%s)", err)
+					log.Errorf("import: could not create default thumbnails (%s)", err.Error())
 				}
 			}