This specific report is from using ACE, but similar reports exist for those who use ACRE2, so it's not a mod-specific issue. See Additional Information for a proposed resolution to the issue from @dedmen

When loading ACE through the steam workshop and launcher, where the path to the folder is as follows:
E:\Mängud\steamapps\common\Arma 3\!Workshop\@ace__ (ignore underscores... just avoiding tagging a user)
Errors are produced for each -x64.dll file that is attempted to be loaded:

12:09:59 [ACE] (common) INFO: ACE is version 3.12.6.43.
12:09:59 [ACE] (common) INFO: CBA is version 3.11.2.190515 (min required 3.9.0)
12:09:59 CallExtension 'ace_break_line' could not be found
12:09:59 [ACE] (common) ERROR: Extension ace_break_line_x64.dll not found. z\ace\addons\common\functions\fnc_checkFiles.sqf:91
12:09:59 CallExtension 'ace_parse_imagepath' could not be found
12:09:59 [ACE] (common) ERROR: Extension ace_parse_imagepath_x64.dll not found. z\ace\addons\common\functions\fnc_checkFiles.sqf:91
12:09:59 CallExtension 'ace_medical' could not be found
12:09:59 [ACE] (common) ERROR: Extension ace_medical_x64.dll not found. z\ace\addons\common\functions\fnc_checkFiles.sqf:91
12:09:59 CallExtension 'ace_clipboard' could not be found
12:09:59 [ACE] (common) ERROR: Extension ace_clipboard_x64.dll not found. z\ace\addons\common\functions\fnc_checkFiles.sqf:91
12:09:59 CallExtension 'ace_fcs' could not be found
12:09:59 [ACE] (common) ERROR: Extension ace_fcs_x64.dll not found. z\ace\addons\common\functions\fnc_checkFiles.sqf:91
12:09:59 CallExtension 'ace_advanced_ballistics' could not be found
12:09:59 [ACE] (common) ERROR: Extension ace_advanced_ballistics_x64.dll not found. z\ace\addons\common\functions\fnc_checkFiles.sqf:91

However, moving the addon (by copying the exact files contained in the workshop folder) to a path without a special character:
E:\ACETest\@ace3test
Loads the addon just fine.