Problem mit der Android app

Hallo Zusammen,
Ich habe seit kurzem das Problem, das die Android app beim download/öffnen von dateien abstürtzt. Kann mir jemand helfe? bin in sowas totaler Änfanger mit fast keiner Ahnung ;-).
Fehler meldung ist folgende:

************ CAUSE OF ERROR ************

javax.net.ssl.SSLHandshakeException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.

at com.google.android.gms.org.conscrypt.ConscryptFileDescriptorSocket.startHandshake(:com.google.android.gms@20104019@20.1.04 (040400-288960190):27)

at okhttp3.internal.connection.RealConnection.connectTls(RealConnection.java:320)

at okhttp3.internal.connection.RealConnection.establishProtocol(RealConnection.java:284)

at okhttp3.internal.connection.RealConnection.connect(RealConnection.java:169)

at okhttp3.internal.connection.StreamAllocation.findConnection(StreamAllocation.java:258)

at okhttp3.internal.connection.StreamAllocation.findHealthyConnection(StreamAllocation.java:135)

at okhttp3.internal.connection.StreamAllocation.newStream(StreamAllocation.java:114)

at okhttp3.internal.connection.ConnectInterceptor.intercept(ConnectInterceptor.java:42)

at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)

at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)

at okhttp3.internal.cache.CacheInterceptor.intercept(CacheInterceptor.java:93)

at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)

at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)

at okhttp3.internal.http.BridgeInterceptor.intercept(BridgeInterceptor.java:93)

at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)

at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.java:127)

at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)

at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)

at okhttp3.RealCall.getResponseWithInterceptorChain(RealCall.java:257)

at okhttp3.RealCall.execute(RealCall.java:93)

at com.nextcloud.common.OkHttpMethodBase.execute(OkHttpMethodBase.kt:126)

at com.nextcloud.common.NextcloudClient.execute(NextcloudClient.kt:67)

at com.owncloud.android.lib.resources.activities.GetActivitiesRemoteOperation.run(GetActivitiesRemoteOperation.java:131)

at com.nextcloud.common.NextcloudClient.execute(NextcloudClient.kt:63)

at com.owncloud.android.ui.fragment.FileDetailActivitiesFragment.lambda$fetchAndSetData$5$FileDetailActivitiesFragment(FileDetailActivitiesFragment.java:331)

at com.owncloud.android.ui.fragment.-$$Lambda$FileDetailActivitiesFragment$N7POe6TZvoGcuV2qP-niPxY_7t4.run(Unknown Source:12)

at java.lang.Thread.run(Thread.java:764)

Caused by: java.security.cert.CertificateException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.

at com.android.org.conscrypt.TrustManagerImpl.verifyChain(TrustManagerImpl.java:661)

at com.android.org.conscrypt.TrustManagerImpl.checkTrustedRecursive(TrustManagerImpl.java:539)

at com.android.org.conscrypt.TrustManagerImpl.checkTrusted(TrustManagerImpl.java:495)

at com.android.org.conscrypt.TrustManagerImpl.checkTrusted(TrustManagerImpl.java:418)

at com.android.org.conscrypt.TrustManagerImpl.getTrustedChainForServer(TrustManagerImpl.java:339)

at android.security.net.config.NetworkSecurityTrustManager.checkServerTrusted(NetworkSecurityTrustManager.java:94)

at android.security.net.config.RootTrustManager.checkServerTrusted(RootTrustManager.java:88)

at java.lang.reflect.Method.invoke(Native Method)

at com.google.android.gms.org.conscrypt.Platform.checkTrusted(:com.google.android.gms@20104019@20.1.04 (040400-288960190):2)

at com.google.android.gms.org.conscrypt.Platform.checkServerTrusted(:com.google.android.gms@20104019@20.1.04 (040400-288960190):1)

at com.google.android.gms.org.conscrypt.ConscryptFileDescriptorSocket.verifyCertificateChain(:com.google.android.gms@20104019@20.1.04 (040400-288960190):7)

at com.google.android.gms.org.conscrypt.NativeCrypto.SSL_do_handshake(Native Method)

at com.google.android.gms.org.conscrypt.NativeSsl.doHandshake(:com.google.android.gms@20104019@20.1.04 (040400-288960190):6)

at com.google.android.gms.org.conscrypt.ConscryptFileDescriptorSocket.startHandshake(:com.google.android.gms@20104019@20.1.04 (040400-288960190):14)

… 26 more

Caused by: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.

… 40 more

************ APP INFORMATION ************

ID: com.nextcloud.client

Version: 30100090

Build flavor: gplay

************ DEVICE INFORMATION ************

Brand: Cat

Device: CatS61

Model: S61

Id: OPM1.171019.011

Product: CatS61

************ FIRMWARE ************

SDK: 27

Release: 8.1.0

Incremental: LTE_D0201121.0_S61_0.060.01