Illustrator Ver.24 startup error occurs when installing Photoshop Ver.26.1 or later.

[assause]

Windows 10/11
Photoshop Ver. 26.1 or later
Illustrator Ver. 24 (confirmed as 24.3)

When Illustrator Ver. 24 is started with these combinations, an unknown error [CANT] is displayed in Illustrator, and then operations such as creating a new file cannot be performed.
Also, a crash report will be displayed when exiting Illustrator.

This problem does not occur in Photoshop Ver. 26.0 or lower.
The same error does not occur at startup in other versions of Illustrator (Ver. 16 to 29).
This problem seems to be caused by Photoshop.

Although Illustrator is an unsupported version, it is still a problem when an application that should have nothing to do with Photoshop stops working properly.

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Original text (Japanese):

Windows 10/11
Photoshop Ver.26.1以降
Illustrator Ver.24（確認は24.3）

これらの組み合わせでIllustrator Ver.24を起動した場合、Illustratorで不明なエラー[CANT]が表示され、その後、ファイルの新規作成などの動作が行えなくなります。
またIllustrator終了時にクラッシュレポートが表示されます。

Photoshop Ver.26.0以下では発生しません。
Illustrator のその他のバージョン（Ver.16～29）においては起動時の同様エラーは起きません。
Photoshopが要因と思われる問題となります。

Illustratorはサポート終了バージョンとは言え、Photoshopとは関係のないはずのアプリケーションが正常動作しなくなるのは問題と言えます。

window10でPhotoshopのバージョン26.1～をインストールするとIllustratorCC2020（24）が起動時エラーが出て正常起動しない

[bluebrad]

Here are some steps you can take. Note: please back up your settings files in your user preferance files on Windows. Normally, these files are located at C:/Users/{username}/AppData/Adobe/. You can rename all Illustrator and Photoshop files with {xphotoshopv26}, then try opening them again. If they work, great—it looks like it was the user's preference files crashing the boot-up. If this still does not fix it, please try uninstalling and using the Adobe Remover Tool. https://helpx.adobe.com/creative-cloud/kb/cc-cleaner-tool-installation-problems.html
after you reinstall rename the user preferances files back to your orginal name and see if they boot. Let me know if you still have any problems or if this worked.

[monokano]

This trouble occurs without exception on macOS.
How do I fix it on macOS?

[bluebrad]

MAC
/Users/<your_username>/Library/Preferences/Adobe (programname) ## Settings/en_US/
you can rename the settings folder by adding an X to the ends and see if relaunch to see if it is working again.

[monokano]

Thanks for the advice. Unfortunately, the cause was not a corruption of the preferences.

[bluebrad]

What are your hardware specks?

[monokano]

This issue occurs on both macOS and Windows, so it is unlikely to be caused by hardware specifications.
The problem only arises in environments where Photoshop 2025 (v26.1) or later is installed. In such cases, only Illustrator 2020 (v24) displays an error upon startup: “an unknown error [CANT]”.