No sure why I was unable to report a bug, and so I have to create this post. I have beent trying to import H.264 video and it consistently fails - tried with latest 15.6 and also 15.4.1 and 15.3.1 (just in case). The details are as follows…

H.264 video import consistently fails with Dynamic Link server error on Apple Silicon Mac

Summary

Lightroom Classic 15.6 on an Apple Silicon Mac cannot import H.264/AVC video.

The import causes dynamiclinkmediaserver to consume approximately 100% CPU. After approximately one minute, Lightroom reports:

An unknown error has occurred while reading the video file. Connecting to Dynamic Link server failed. (1)

The same H.264 file can be successfully processed by Apple's avconvert, demonstrating that the file itself is readable by the macOS media framework.

A Dynamic Link process sample captured during the failure shows the server spending its time in Adobe's mc_dec_avc AVC/H.264 decoder path.

System information

Lightroom Classic: 15.6 [202609251514-f839cde9]
License: Creative Cloud
Operating system: macOS 27.0.1 [26A434]
Application architecture: arm64
Processor: Apple M6
Memory: 24 GB
GPU: Apple M6 / Metal
Internal Camera Raw: 18.7 [2743]
Dynamic Link Media Server: 24.0.0
MainConcept AVC decoder: mc_dec_avc version 13.2
MainConcept AVC architecture: universal binary containing arm64 and x86_64

Steps to reproduce

1. Launch Lightroom Classic 15.6.

2. Create or open a catalog.

3. Choose Import.

4. Select an H.264/AVC video file.

5. Start the import.

6. dynamiclinkmediaserver starts consuming approximately 100% CPU.

7. Wait approximately one minute.

8. Lightroom reports:

   "An unknown error has occurred while reading the video file. Connecting to Dynamic Link server failed. (1)"

9. The video is not imported.

The problem is reproducible using both Fujifilm X-T2 H.264 video and an H.264 video converted from an iPhone source.

Critical control test — HEVC works

HEVC video imports successfully into the same Lightroom Classic installation.

Specifically:

• iPhone 1080p HEVC/Dolby Vision → imports successfully

• iPhone 4K HEVC/Dolby Vision → imports successfully

• Fujifilm-derived 4K HEVC test file → imports successfully

Therefore this does not appear to be a general video-import problem or a general 4K problem.

H.264 from different sources produces the same failure

The original Fujifilm X-T2 footage is:

• 3840 × 2160

• 24 fps

• H.264 High Profile

• approximately 100 Mbps

• 8-bit 4:2:0

• PCM audio

However, the same Lightroom failure also occurs with an H.264 file converted from an iPhone video.

Therefore the problem does not appear to be specific to Fujifilm's encoder, camera metadata, or Fujifilm video files.

Lightroom version comparison

The same H.264 test file was tested in separate fresh test catalogs using:

• Lightroom Classic 15.3.1 → FAIL

• Lightroom Classic 15.4.x → FAIL

• Lightroom Classic 15.6 → FAIL

Dynamic Link Media Server remained version 24.0.0 across these Lightroom versions.

The problem therefore persists across multiple Lightroom Classic releases and is reproducible in the current release, 15.6.

Container and audio tests

The Fujifilm H.264 source was also tested after:

• remuxing into a new MOV container

• remuxing into MP4

• removing the audio stream

• re-encoding the H.264 video

• reducing the video resolution to 1080p

The H.264 import problem persisted in the tested variants.

This makes the original MOV container and PCM audio unlikely to be the fundamental cause.

macOS media framework control test

The exact H.264 test file that Lightroom cannot import was successfully processed using Apple's built-in avconvert utility.

Result:

avconvert completed with error:0

This demonstrates that the H.264 file can be successfully read and processed by Apple's macOS media framework.

Dynamic Link process evidence

A 5-second process sample of dynamiclinkmediaserver was captured while Lightroom was attempting to import the failing H.264 video.

The relevant call path enters:

ServerImpl::StartImport
→ FileManager::ImportFile
→ ML::ImporterFactory::CreateFileMediaInfo
→ ImporterMPEG::ImportEntry
→ MPEGInstance::Open
→ mpegInOpen
→ mc_mfimport
→ sessionDefault (in mc_dec_avc)

The CPU samples are concentrated in the mc_dec_avc portion of the call stack.

This appears to indicate that Dynamic Link is reaching the Adobe/MainConcept AVC decoder and becoming stuck while processing the H.264 stream, rather than simply failing to communicate with Lightroom.

Adobe component information

The Dynamic Link application contains:

mc_dec_avc.framework/Versions/13/mc_dec_avc

The component reports:

Version: 13.2

The binary is a universal Mach-O binary containing:

• x86_64

• arm64

The Dynamic Link Media Server itself is also a universal binary and runs natively as ARM64 on this Apple Silicon system.

Expected result

Valid H.264/AVC video should import successfully into Lightroom Classic.

Actual result

Dynamic Link Media Server consumes approximately 100% CPU while processing the H.264 video and eventually Lightroom reports:

"An unknown error has occurred while reading the video file. Connecting to Dynamic Link server failed. (1)"

Additional observations

The problem is highly reproducible.

It occurs with H.264 files from different sources.

HEVC video works normally.

The same H.264 file can be successfully processed by Apple's avconvert.

The failure persists across Lightroom Classic 15.3.1, 15.4.x, and the current 15.6 release.

Request to Adobe

Please investigate the AVC/H.264 decoding path used by Dynamic Link Media Server 24.0.0, particularly the mc_dec_avc 13.2 component, on Apple Silicon Macs running macOS 27.

I can provide the following diagnostic material if required:

1. Original Fujifilm X-T2 H.264 sample

2. iPhone-derived H.264 sample that reproduces the problem

3. Working iPhone HEVC control sample

4. Dynamic Link 5-second process sample

5. Lightroom system information

6. Additional logs or diagnostic files as requested