[Bug]: Nextcloud Client makes Windows Explorer non-responsive when numerous sync errors occur

[siblount]

⚠️ Before submitting, please verify the following: ⚠️

• This is a bug, not a question or a configuration issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server and Desktop Client are up to date. See Server Maintenance and Release Schedule and Desktop Releases for supported versions.
• I agree to follow Nextcloud's Code of Conduct

Bug description

The immediate issue is that the Nextcloud Client is causing Windows Explorer to be non-responsive (freeze) that then crashes. This only occurs when there are numerous sync errors. By Windows Explorer, I really mean the taskbar, since the desktop, taskbar, and actual Windows Explorer are all connected.

When this freezing behavior occurs, a noticeable side-effect is that the Nextcloud notification icon becomes invisible (see image below). Please don't tell me the tray is re-rendering infinitely (tell me). As you see below, that invisible slot is actually for Nextcloud.

Another side-effect is that the Nextcloud UI is extremely slow, unresponsive, and has a very slow UI feedback (~3-5 sec feedback time).

This seems to only happen when there are a lot of errors and the server is still trying to provide sync updates. For some reason, I am getting weird 401 or 499 errors but that's not the problem.

I guess once Nextcloud stops adding error elements or whatever, it goes back to its usual refreshing flickering madness. If even more errors occur, well. We're back to square one. Fun fact: It also crashes Windows Snippet Tool when trying to record it 😂

nextcloud_flickering_ui_icon.mp4

Pausing the sync immediately stops this madness as you will see in the logs.

LOGS:
logs.txt
logs1.txt

(I am going to double check if this still exists with the latest Desktop Client update 3.15.1 momentarily)

Steps to reproduce

1. Bulk upload files (let's say 100+).
2. After a few moments, observe state of the taskbar.

Expected behavior

Desktop client does not cause Windows Explorer to freeze, ultimately leading to a crash.

Which files are affected by this bug

NA

Operating system

Windows

Which version of the operating system you are running.

Windows 11 23H2 Build 22631.4317

Package

Official Windows MSI

Nextcloud Server version

30.0.2

Nextcloud Desktop Client version

3.15.0

Is this bug present after an update or on a fresh install?

Updated from a minor version (ex. 3.4.2 to 3.4.4)

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

Are you using an external user-backend?

• Default internal user-backend
• LDAP/ Active Directory
• SSO - SAML
• Other

Nextcloud Server logs

Additional info

Using virtual files in a local folder, total file upload was around 10 GB.

This may be related to #6749.

[siblount]

I can't verify if this is still an issue on 3.15.1 due to #7632 - it's not giving me any indication of syncing. Maybe I need to let this run for a bit?

EDIT: Yeah, just letting it run for a LONG time. It still exists for 3.15.1. I did a file upload of 5000 files with less than 100 MBs.

[tomdereub]

I think I have the same issue now with 3.15.3 and windows 10. I modified quite a lot of files, and now windows explorer is stuck.
On task manager, windows explorer is stuck with 15% CPU (one full core), but nextcloud is quite low (about 2-3% CPU).
As a workaround, I manually restarted explorer.exe. It's responsive again, but stays high on CPU (15-16%), for about 10 minutes, then nextcloud managed to finish sync.

[Anthchirp]

Can confirm this behaviour on 3.16.2 Windows 11. Server 30.0.8. I have 3769 files that refuse to sync for network error 499, and am also seeing the tray icon flickering and the explorer process hang and crash.

ETA: As suggested above I left it running/hanging for a long time, and after a few rounds of "Failed to sync files" errors it cleared itself up.

[Rello]

With the upcoming release of the desktop client (version 3.16.3), the bulk upload feature will be temporarily disabled.

We understand that some of you may rely on this feature, and this decision wasn't made lightly. However, we've encountered ongoing technical issues involving the interaction between the client, server, and underlying systems—including network stack and Qt dependencies—that impact the stability of the feature.

In prioritizing a smoother and more reliable experience for all users, we believe it's best to disable bulk upload for now. The overall benefit of the feature does not currently outweigh the potential for disruption.

If you're unable to update to version 3.16.3 right away, we recommend manually disabling the feature on the server side to avoid any issues:
🔧 How to disable bulk upload on the server →

We appreciate your understanding and are continuing to work on improving the experience.