In recent versions of After Effects, I'm unable to install or sign new .zxp files

Bug Report: New CEP Extensions Fail to Load — "Unsupported Manifest version

Summary

Any newly-installed CEP extension fails to load in After Effects, regardless of manifest content, signing status, or installation method. The failure occurs at the earliest stage of manifest parsing — before PlugPlug even logs that it has begun reading the file — while dozens of previously-installed, third-party CEP extensions continue to load and function normally in the same session.

Environment
App: Adobe After Effects 26.3
Affected versions: 26.3.0, 26.2.1, and 25.6.5 (all report PlugPlug version : 12.0.1.2 in the CEP log — i.e. all three run the identical CEP 12 engine)
OS: macOS 26.5.2
CEP debug mode: Enabled correctly for CSXS.12 (both via manual defaults write com.adobe.CSXS.12 PlayerDebugMode 1 and via the aescripts ZXP/UXP Installer's built-in debug toggle)

Symptom
A new CEP extension does not appear under Window > Extensions (Legacy) after installation, no matter how it was installed (manual folder copy in debug mode, or a properly signed .zxp installed via the ZXP/UXP Installer app). 

Exact error (from CEP12-AEFT.log)
ERROR An illegal argument occurred while creating the ExtensionManifest: Unsupported Manifest version '' in '/Users/<user>/Library/Application Support/Adobe/CEP/extensions/<bundle>/csxs/manifest.xml'

 

Note the empty string in Unsupported Manifest version '' — this occurs even when the manifest's Version attribute clearly contains a valid value (tested with both "9.0" and "12.0"; identical error both times, including identical timestamp-adjacent log text). This suggests the logged message does not actually reflect the parsed value, and the true failure occurs earlier in parsing than the error text implies.

Key diagnostic detail: every other extension in the log — over 30 of them, including third-party tools like Bodymovin, MoBar, and others — logs a Parsing manifest / ---------------- header line before any content is read, even when that extension later fails for an unrelated reason (e.g. unsupported host). Our extension's manifest never reaches that log line at all — the error is thrown before parsing is reported to have started, for every attempt, across every variation tested below.

Steps to reproduce

1. On a machine with AE 25.x or 26.x installed (CEP 12.0.1.2), confirm that existing CEP extensions load normally (Window > Extensions (Legacy) shows populated entries, and/or CEP log shows multiple Adding extension with id... lines).
2. Create a new CEP extension folder in ~/Library/Application Support/Adobe/CEP/extensions/ (or the system-level equivalent) with a manifest built from Adobe's own official CEP Extension manifest documentation/example.
3. Enable CSXS.12 debug mode (defaults write com.adobe.CSXS.12 PlayerDebugMode 1).
4. Restart After Effects.
5. Observe: the new extension does not appear in Window > Extensions (Legacy), and the CEP log shows the error above.

Variations already tested (all produce the identical error)

What this rules out

• Manifest content/attribute values
• Manifest schema version number specifically
• Attribute ordering
• Code signing / trust status
• Debug-mode configuration
• File encoding / hidden character corruption
• Gatekeeper quarantine attributes
• AE version (reproduced identically across three releases)
• Whether CEP functions at all on this machine (it clearly does — many pre-existing extensions load without issue)

Suspected root cause

Given that only newly installed extensions fail, while extensions that were presumably already validated/cached prior to some point continue to load successfully, this looks like a regression in PlugPlug 12.0.1.2's manifest validation path specifically for extensions being registered for the first time, rather than a problem with any specific manifest file.

Attachments to include when filing

• CEP12-AEFT.log (the session log showing the exact error, plus the full successful-extension list for comparison)
• The manifest.xml that fails to load
• The minimal-reproduction manifest.xml (copied from Adobe's own docs) that also fails identically