How to reproduce : 

Add following files : 

Aé$çèv+tés,et (blanc) #agglo3 ! hgd ' gs÷b.docx, .xlsx, .ppt

For those files, miniatures are not displayed. 

Following error is present in logs : 
Date:            Mon Jan 31 18:39:01 CET 2022 (1643650741275)Thread:        ametys-async-observe-2
Message #:    4300
Level:        ERROR
NDC:            
Category:    org.ametys.plugins.workspaces.documents.onlyoffice.OnlyOfficeManager
Message:        An error occurred generating png file with uri 'https://onlyoffice.ametys.org/cache/files/conv_68101fea-08d1-41cb-8515-09e2a450ccc0.2022.01.31.18.38.00_png/output.png/A%EF%BF%BD%24%EF%BF%BD%EF%BF%BDv%2Bt%EF%BF%BDs%2Cet%20(blanc)%20%3B%23agglo3%20!%20hgd%20'%20gs%EF%BF%BDb.png?md5=Be36byR3M9_RPCABR3EpmA&expires=1643651642&filename=A%EF%BF%BD%24%EF%BF%BD%EF%BF%BDv%2Bt%EF%BF%BDs%2Cet%20(blanc)%20%3B%23agglo3%20!%20hgd%20'%20gs%EF%BF%BDb.png'Location:    org.ametys.plugins.workspaces.documents.onlyoffice.OnlyOfficeManager._generatePNGFileInCache(OnlyOfficeManager.java:416)
Thrown:
java.io.IOException: Server returned HTTP response code: 403 for URL: https://onlyoffice.ametys.org/cache/files/conv_68101fea-08d1-41cb-8515-09e2a450ccc0.2022.01.31.18.38.00_png/output.png/A%EF%BF%BD%24%EF%BF%BD%EF%BF%BDv%2Bt%EF%BF%BDs%2Cet%20(blanc)%20%3B%23agglo3%20!%20hgd%20'%20gs%EF%BF%BDb.png?md5=Be36byR3M9_RPCABR3EpmA&expires=1643651642&filename=A%EF%BF%BD%24%EF%BF%BD%EF%BF%BDv%2Bt%EF%BF%BDs%2Cet%20(blanc)%20%3B%23agglo3%20!%20hgd%20'%20gs%EF%BF%BDb.png    at java.base/jdk.internal.reflect.GeneratedConstructorAccessor222.newInstance(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
    at java.base/sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1963)
    at java.base/sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1958)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1957)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1525)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1509)
    at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:245)
    at org.apache.excalibur.source.impl.URLSource.getInputStream(URLSource.java:284)
    at org.ametys.plugins.workspaces.documents.onlyoffice.OnlyOfficeManager._generatePNGFileInCache(OnlyOfficeManager.java:396)
    at org.ametys.plugins.workspaces.documents.onlyoffice.OnlyOfficeManager.generateThumbnailInCache(OnlyOfficeManager.java:299)
    at org.ametys.plugins.workspaces.documents.observers.FileThumbnailObserver.observe(FileThumbnailObserver.java:103)
    at org.ametys.core.observation.ObservationManager$ParallelAsyncObserve._observe(ObservationManager.java:496)
    at org.ametys.core.observation.ObservationManager$AbstractAsyncObserve.call(ObservationManager.java:442)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.io.IOException: Server returned HTTP response code: 403 for URL: https://onlyoffice.ametys.org/cache/files/conv_68101fea-08d1-41cb-8515-09e2a450ccc0.2022.01.31.18.38.00_png/output.png/A%EF%BF%BD%24%EF%BF%BD%EF%BF%BDv%2Bt%EF%BF%BDs%2Cet%20(blanc)%20%3B%23agglo3%20!%20hgd%20'%20gs%EF%BF%BDb.png?md5=Be36byR3M9_RPCABR3EpmA&expires=1643651642&filename=A%EF%BF%BD%24%EF%BF%BD%EF%BF%BDv%2Bt%EF%BF%BDs%2Cet%20(blanc)%20%3B%23agglo3%20!%20hgd%20'%20gs%EF%BF%BDb.png    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1913)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1509)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getHeaderField(HttpURLConnection.java:3084)
    at java.base/java.net.HttpURLConnection.getHeaderFieldDate(HttpURLConnection.java:599)
    at java.base/java.net.URLConnection.getLastModified(URLConnection.java:566)
    at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.getLastModified(HttpsURLConnectionImpl.java:413)
    at org.apache.excalibur.source.impl.URLSource.getInfos(URLSource.java:164)
    at org.apache.excalibur.source.impl.AbstractSource.checkInfos(AbstractSource.java:62)
    at org.apache.excalibur.source.impl.URLSource.getInputStream(URLSource.java:227)
    ... 9 more