[FIX + BUG REPORT] Avast Secure Browser Won’t Start – ProcessSingleton Error Code 5 / Corrupted User Data Profile
I am posting this both as a workaround for affected users and as a technical bug report for the Avast Secure Browser development team.
After extensive troubleshooting, I was able to get Avast Secure Browser working again without reinstalling Windows and without deleting my old browser profile.
Symptoms
Avast Secure Browser suddenly stopped launching.
Clicking AvastBrowser.exe produced no visible browser window.
During troubleshooting, I also encountered a Windows error involving:
chrome_elf.dll
with Windows status:
0xc000012f
However, this DLL error should not necessarily be considered the root cause. Further testing produced a much more relevant error related to the browser profile and Chromium’s ProcessSingleton mechanism.
Important: Do Not Immediately Delete or Reinstall Everything
If Avast Secure Browser suddenly stops launching, I recommend not repeatedly uninstalling/reinstalling the browser and not deleting the User Data folder before backing it up.
That folder may contain important data such as:
Technical Diagnosis
I launched Avast Secure Browser from PowerShell with logging enabled.
The important error was related to:
ProcessSingleton
with:
Error code: 5
and:
Failed to create a ProcessSingleton
This strongly suggested that Avast Secure Browser/Chromium was unable to create or access the required lock/singleton mechanism inside the existing browser profile.
The affected profile was located at:
%LOCALAPPDATA%\AVAST Software\Browser\User Data
Confirmation Test
A very useful diagnostic step was launching Avast Secure Browser with a different temporary user-data directory.
For example, the browser was tested using a temporary profile with:
--user-data-dir="$env:TEMP\AvastTestProfile"
This is important because it allows the browser executable to be tested separately from the existing user profile.
If Avast Secure Browser works with a new/temporary profile but does not work with the original profile, this strongly points toward a problem inside the original User Data environment rather than simply a broken Avast executable.
The Workaround That Fixed It
Press:
Windows + R
Enter:
%LOCALAPPDATA%\AVAST Software\Browser\
Find:
User Data
DO NOT DELETE THIS FOLDER.
Rename it to:
User Data OLD
Then launch Avast Secure Browser normally.
In my case, Avast Secure Browser automatically created a fresh:
User Data
directory.
The browser then started working again.
Why This Is Interesting for Avast Developers
The sequence observed was:
Existing User Data profile
→ Avast Secure Browser fails to start
→ Chromium/Avast logging reports ProcessSingleton failure with Error code: 5
→ Browser works when isolated from the original profile
→ Rename original User Data
→ Avast creates a clean User Data
→ Browser starts normally
This suggests that the failure is associated with the existing profile environment, potentially involving profile locking, permissions, stale/corrupted singleton state, or another condition preventing ProcessSingleton initialization.
I would not claim to know which internal component is ultimately responsible, but the behavior appears reproducible enough to be useful for investigation.
Potential Product Improvement
From a user-experience perspective, Avast Secure Browser could potentially handle this condition more gracefully.
Instead of apparently doing nothing when the existing profile cannot initialize, the browser could detect a ProcessSingleton/profile initialization failure and display a recovery message such as:
“Avast Secure Browser could not access your browser profile. Would you like to start with a temporary profile or repair the existing profile?”
Ideally, this recovery mechanism would preserve the original profile automatically before making changes.
This would prevent users from repeatedly reinstalling the browser or deleting potentially valuable browser data while trying to solve the problem.
IMPORTANT: Recover Your Data Carefully
Keep:
User Data OLD
until you have verified that everything you need is available in the newly created profile.
Check:
-
Bookmarks
-
Passwords
-
History
-
Settings
If something is missing, recover only the necessary data from the old profile or another backup.
Do not blindly overwrite the new working User Data directory with the complete old directory, because that may reintroduce the condition that prevented Avast Secure Browser from starting.
For Avast Developers
The most useful diagnostic indicators from this case appear to be:
ProcessSingleton
Failed to create a ProcessSingleton
Error code: 5
Affected profile:
%LOCALAPPDATA%\AVAST Software\Browser\User Data
Successful isolation test:
--user-data-dir="$env:TEMP\AvastTestProfile"
Successful workaround:
User Data → User Data OLD
followed by allowing Avast Secure Browser to generate a fresh profile.
I can provide the original screenshots/log output if useful for reproducing or investigating the issue.
2. TRY THIS PROCEDURE
IF AVAST SECURE BROWSER OPENS AGAIN
DO NOT DELETE 
The important point