I confirm this bug on Redmi 12C for .docx files, elaboration is available in another thread.
Your other thread describes the situation with specific txt files. I believe this topic provides a different scenario. Let’s wait for information from @Anyum
Alright, thanks for increasing my forum level. And this is my screen record showing my attempt to open my PPTX file saved on my device (I’ve also have another screen record showing an attempt to open an XLSX file saved on my device too)
Also, this is my screen record showing an attempt to create a new DOCX file. I’m really stuck at “Preparing to open” screen even after two minutes.
Here’s what I’ve found when I’m trying to open my XLSX file stored on my device
Indeed other topics I created here at this moment are related to the .txt files but I’m taking about the topic I didn’t created but commented in. Here is what I wrote in the topic Files from phone can only be opened inside OO app and not from outside OO app when we choose an option "Open with" regarding the situation with .docx and .xlsx and .pptx files:
‘ONLYOFFICE Sample Spreadsheets.xlsx’ opened from ‘open with other app’ list: app immediately closed after showing ‘Preparing to open’ message window. Another attempt displayed ‘Wanna repair?’ dialogue window, the results of the repair was even quicker exit of the app.
‘ONLYOFFICE Sample Presentation.pptx’ opened from ‘open with other app’ list: same as the previous.
‘ONLYOFFICE Sample Document.docx’ opened from ‘open with other app’ list: the ‘Preparing to open’ message window never disappeared, the OnlyOffice app hunged.
The very same results I’m getting now when opening that files from inside of the app. It looks as the problem with the app handling the cache / the storage. But on Redmi 5 the files were successfully opened from inside of the OnlyOffice app.
But @Anyum already answered, his feedback is a prio.
Hello @Anyum and @dimm
@Anyum Thank you for provided data, we are checking the situation.
@dimm, ah, now I see it. Your feedback was collected within the mentioned post, the checks are still in progress. I believe these are separated cases, because in your mentioned thread files open via File Manager. I will contact you in this thread when we have something to share.
Hello again @Anyum
Please confirm that described situation reproduces constantly (100% of your attempts of file opening).
Yes, this problem occurs constantly, even after several reinstalls of the app.
Thank you, we are checking the situation.
I encountered this with OO on OnePlus 10 Pro.
The issue went away when I cleared cache with Settings > Clear cache.
Hello @mj-j
Thank you for your sharing.
@Anyum just for your information, we are still investigating this case.
Thanks for the information 
Hello @Anyum
Please confirm that the situation reproduces on the latest version of Android app (v.7.5).
Yes, this situation is still happen on the latest version (7.5)
When opening all of sample documents and opening any file stored on my device (either from the file manager or from the application), now the app crashes after displaying “Preparing to open” for several seconds.
Clearing app cache and force stop then reopen still not fixing the problem.
My screen record showing an attempt to open the sample document
Thank you for the provided details, we are looking into it.
Have you found the resolution, I am facing the same issue on android , desktop version works ok and sample files also work.
after Preparing to open
I get message
Connection lost the viewer mode is enabled
Than nothing happens, pls suggest the resolution
Hello @Anyum
We have released v.8.0 for Android. Could you please double-check the situation on the latest version?