Failed to process at step 3 ''Merge orthomosaic tiles failed"

[Processing]: Substep Ortho load pcl started.

[Processing]: Load point cloud

[Processing]: Substep Ortho load pcl finished.

[Processing]: Substep Ortho gridded dsm started.

[Processing]: Point cloud gridding

[Processing]: Substep Ortho gridded dsm finished.

[Processing]: Substep Ortho dsm started.

[Processing]: Generate DSM

[Processing]: Substep Ortho dsm merge started.

[Processing]: Merge DSM tiles

[Processing]: Substep Ortho dsm merge finished.

[Processing]: Substep Ortho dsm finished.

[Processing]: Substep Ortho dtm started.

[Processing]: Downsampling

[Processing]: Generate DTM Classification Mask

[Processing]: DTM generation

[Processing]: Generate DTM Classification Mask

[Processing]: DTM generation

[Processing]: Generate DTM Classification Mask

[Processing]: DTM generation

[Processing]: Substep Ortho dtm merge started.

[Processing]: DTM merge

[Processing]: Substep Ortho dtm merge finished.

[Processing]: Substep Ortho dtm finished.

[Processing]: Substep Ortho contours from dtm started.

[Processing]: Generating Contour Lines from DTM

[Processing]: Downsampling

[Warning]: Warning w9020: GDAL Error \<1\> DXF layer does not support arbitrary field creation, field 'ID' not created.

[Warning]: Warning w9020: GDAL Error \<1\> DXF layer does not support arbitrary field creation, field 'Elevation' not created.

[Processing]: Substep Ortho contours from dtm finished.

[Processing]: Substep Ortho mosaic started.

[Processing]: Loading ortho images [group1]

[Warning]: Ortho gain: cannot load precomputed gain data. It will be computed.

[Processing]: Preparing color balancing [group1]

[Processing]: Color balancing [group1]

[Processing]: Generate orthomosaic [group1]

[Processing]: Merge orthomosaic tiles [group1]

[Warning]: Warning w9020: GDAL Error \<1\> LZWDecode:Corrupted LZW table at scanline 0

[Warning]: Warning w9020: GDAL Error \<1\> TIFFReadEncodedStrip() failed.

[Warning]: Warning w9020: GDAL Error \<1\> D:/VRR T7/3\_dsm\_ortho/2\_mosaic/tiles/VRR T7\_transparent\_mosaic\_group1\_2\_5.tif, band 3: IReadBlock failed at X offset 0, Y offset 0

[Warning]: Warning w9020: GDAL Error \<1\> GetBlockRef failed at X block offset 0, Y block offset 0

[Error]: Merge tiles: failed to load tile!

[Error]: Merge orthomosaic tiles [group1] failed!

[Error]: \>\>\>Merge orthomosaic tiles [group1]\<\<\< failed with error (failed to run a step!)!

[Error]: \>\>\>Merge orthomosaic tiles [group1]\<\<\< failed with error (failed to run a step!)!

[Processing]: Substep Ortho mosaic finished.

[Processing]: Step2Done

Hi, I would suggest you write us a support ticket as we would need the full log file and quality report. Did you ever face the same issue with other projects?

Are you processing the project locally? Processing on an external hard drive is slower than on the local system hard drive and it can also create issues due to connection problems. How much space do you have in the external drive? Is it an SSD? 
Could you please try to process locally and let me know if the problem persists?
You can use the Save Project As… option to copy the project and the results in a local drive, like D, and process only step 3.